Yes the Mk2's were BB, but as you say they were geared very differently. Figure it was a 4 speed box with 4th geared to go over 200. The lower gears were tall, it was all tall gearing for racing at lemans and speeds below 70 or so were irrelevant.
An aluminum 351 is a really nice "exotic" piece. A cam will certaily change things. But will the cam work with your heads, you can go hydraulic if you are rev limited to 6500 rpms anyway.
Destroking may require different rods etc. But if I were to build a 351 based motor it would be aluminum with twm as you have, and aroumd 400 Ci or less, so its a smooth rever. Considder that many a crate 302 these days is an iron 363 which is a lot of stroke, so a stock stroke big bore aluminun 351 is going to be better and smoother. To me if youa re not interested in racing this is the best setup for a GT40. Cant say i am an expert but did spend years thinkign through every possible configurtion.
The only reason I went to a short stroke big bore iron 302 was the possibility of vintage racing, and I do love the sound of a motor at 7k rpm. But for street an aluminum 351 of 400ci or less is the ideal ticket IMHO.
Plus if you keep solids then you ca rev to 7k or more, which would make sense of the 4.22 rear.
As an example for my 302 car the rear is a little shorter than "stock" at 4.0 1st through 3rd are taller than stock 4th stock and 5th shorter. This is based on the torque and rev range of my expected motor, with easy rev drops based on a track scenario.
From what I can see with a 4.22 rear and stock ratios your 1-3 gears are really short, 4th shortish and 5th still pretty tall for highway. Any motor from 450hp up is going to give a lot of pop and your limit in 1-3 is traction. If it were me and I were keeping the box, I would destroke and change cam on the aluminum 351 or go 302.
Either way to do a cam you are pulling the motor and its a lot of work, so why just stop at the cam. With your gears you want a motor where the peak of the powerband is moved a bit up in the rev range and the ability to rev, and a smooth linear ramp up so you can modulate well. A broader soother power delivery making the car way more driveable.
Basicaly a destrokes 351/427 or a worked 302 are not going to be hugely different, but an aluminum block 351 is a very cool exotic piece.
You only live once make th motor right for the car, why suffer with the worng thing. Destroke and re-cam and you are set. 3K in parts and some shop time.