Both of your gearboxes come from Panteras; your Dash-1 is from a 1971 Pantera and your Dash-2 is from a later car (or at least, both wear Pantera gearbox part numbers). Because both are Pantera gearboxes, both will require flipping to be run in a GT40 orientation. I don't know the vintage of your later gearbox, but '71-74 Ford-era Pantera gearboxes did not have any identification tag on the case, so yours is either a 1980s-vintage Pantera gearbox, or perhaps it came from some other car. The non-Pantera (Chevy?) bellhousing would tend to suggest that perhaps it was originally fitted to some other car, i.e. somebody who needed a Pantera-style gearbox could have purchased it directly from a ZF source. There's no way of telling.
I presume your friend's McLaren project will run in the GT40 orientation also?
Although there is nothing specifically 'wrong' with either style of gearbox, generally the Dash-2 is considered superior in most ways--stronger, more robust, and with much greater parts supplies. The case is physically a bit larger and heavier, so you're right--the Dash-1 architecture and physical dimensions are identical to the original GT40 gearbox and thus are more 'right'.
Note that your Dash-1 is setup with cast iron side covers with bolt bosses that splay outwards at a 45-degree angle. The gearbox case itself is only secured to the bellhousing around the periphery of the front case opening with four bolts; the original GT40 bellhousing (along with the Dash-1 Pantera bellhousing, and Mangusta bellhousing) had provisions for two more long bolts to engage these holes in the side plates.
I've seen several original-style GT40 bellhousings on the market (Safir, Gelscoe, Cushman), which have bosses to accept these bolts which are not machined, so you would likely have to open those holes up. The diameter of a Dash-1 and Dash-2 case mouth is different, so the bellhousings won't simply interchange. The back of the bellhousing would need to be machined to mate with the front of the gearbox case.
Also, the Dash-1 input shaft is shorter and uses a different spline pattern. In the Pantera, it didn't actually reach the back of the crankshaft, which was more than a little inconvenient; instead, a pilot bearing adapter had to be used which ran in between the pilot bearing and the snout of the input shaft. I confess I don't know how the system works on the GT40, but would presume that the front-to-back dimension of the bellhousing is shorter, and thus the Dash-1 would engage the back of the crankshaft properly; in that case, the Dash-2 would likely be too long and some cleverness would be required.
Perhaps the Dash-2 GT40 bellhousings have thought of that, and are slightly longer? I really don't know, but need to get smart on this stuff one of these days.
Allow me to cheat a little bit, and poach from a long-ago post on a similar subject that I made elsewhere on this forum:
I know of two sources for GT40 clutch bits, and suspect a third:
1) Gelscoe, in the UK
2) Safir, in the USA
3) Jay Cushman, in the USA (although he doesn't list these parts on his website--he may produce them, or may source them from someplace else? Jay?)
Safir has cast up an original-style bellhousing (I don't know if it's "original-original" or a later design, but it is visibly different from the Gelscoe offering), but uses a super-modern (and expensive) multi-disc clutch from McLeod.
Here are some photos of the Safir setup:
Meanwhile, here's a photo of the Gelscoe bellhousing. Compare it to the Safir unit above; the Safir bellhousing seems to have an integral cooling air scoop or something on the bottom of it? If it hangs down low, I suspect it would become a skidplate before long. The Gelscoe bellhousing has vents on the top and bottom:
Both units seem to be sized (and drilled) for the more common Dash-2 ZF gearbox. The Dash 1 and Dash 0 use only four bolts instead of six (plus two lateral bolts), and the opening is physically smaller.
Have you seen the ERA instruction manual to flip a Pantera gearbox to GT40 orientation? The ZF manual doesn't address this procedure at all, for it presumes that whatever gearbox you are working on is coming out of a car, getting rebuilt and going right back into the same car, thus it's just a traditional overhaul manual, rather than a modification manual. The ERA manual leads you in the right direction to perform the modifications necessary.
The ZF is a very sophisticated and finicky piece of kit. Even people with lots of experience rebuilding transmissions can soon find themselves running out of talent when trying to cope with one of these transaxles, so if this happens to you, there's absolutely no shame in it. Lloyd Butfoy told me that a substantial percentage of the gearboxes he gets in for maintenance come to him in pieces, where owners or even transmission specialists blew the gearbox apart and then quickly found themselves in over their heads.
Since you're in New Zealand, you're in great luck. There are only a handful of people worldwide who are certified as specialists in these animals, and one of them is right there next to you. Paul Donderwinkel is his name, and his e-mail address is firstname.lastname@example.org
(Paul emigrated from London some years back, and has the distinction of owning a very special Pantera, built as a prototype in the 1980s for Carroll Shelby, and powered by a twin-turbo Dodge from new!)
Even if you are determined to pursue the DIY route, I would still touch base with him. He will be a great ally, is likely to have parts on hand, and if not, he can get them easily enough from Lloyd Butfoy as he receives regular shipments. I would strongly urge you to have him take a look at your gearbox, because now is the time to find out if your 2nd gear synchro is trashed, or whatever. Although it's certainly fun and rewarding to perform ZF surgery on your own, it's also filled with the prospects for screwing up something extremely expensive and difficult to replace (particularly with the Dash-1), and sometimes it pays to have some expert advice--or just to hand the gearbox over with instructions to go through it and make it perfect.
Keep us posted and let us know how it turns out!