Well I thought that I would put my .02 cents in here.
The ATB or torque biased differential is a great street diff, however, does not lend itself well to the track as there is not lockup on the decel side of the differential.
As to changing out the 1st gearset, this can not be easily done with a G50/01, as the 1st gearset is intregal to the mainshaft. You would either need to pick up a two-piece mainshaft with the new first gear, or have one welded on. (I can help you with that here.)
As to what is possible, well that all depends on what your budget is.
Cryogenic treatment of the internals will give you much longer life to the gears.
A billet side plate might not be a bad idea, however with only 400 HP I feel that this option is a bit overkill.
I would caution you however with regard to the differential. Regardless of if you go with the ATB or the more traditional plate type differential, these are NOT plug and play upgrades. You will need to make sure that the ring and pinion is set up correctly, otherwise you could possibly do damage to the gearbox, and potentially lock it up completely, in a worst case scenario.
I would do some case work though, make sure to open up new oiling passages to ensure that your gearoil flows correctly front to back. Keep in mind that the biggest killer of gearboxes is heat. I would go so far as to recommend that you install cooler in and out ports (8 AN) and use a good oil cooler and pump, again I can help here too.
The G50 is a great option for mid-engine use, we build many of these for these types of applications, usually we change out the 4th and 5th gear ratios with taller gearing that is better suited to the V8 application.
I would be happy to discuss this in more detail with you should you have some time. Feel free to contact me with any questions that you might have. I am always here to help.
Warmest Regards, and happy building.
Erik Johnson
Carquip Sales
(303) 443-1343 ext 2
(720) 980-9407 cell
PS Make sure that you use a good gearoil, either Swepco 210 or LE 9920 would be my recommendations.