Timbo
Busa in an SV the extra chassis width compared to an S3 should give you lots of scope to pick the optimum engine location and the route for the exhaust.
I would speak direct to Bruce at Arch to sort out any chassis mods he's a top man and really helpful.
I have gone for the Bailey Morris 2 piece prop shaft which suits the location of my engine, the rubber drive flange which I tried from Nova Racing (great quality) needed an almost perfect flange to flange line up, but again with the room you have you could use either options.
Reverse boxes do seem to add in some extra backlash to the drive train, memory's of stripping and rebuilding a Westfield type was interesting...... I have seen the sprocket added to the diff flange which is then hooked up to a starter motor to give reverse, this may need to occupy the same space as the passenger seat as its very tight for room in this location or again it is on the S3.
Gear change choice I have used a stick type on both a Westfield and a Radical, the Westfield used rods and bell cranks and the Radical a cable. For me its got to be a paddle shift and to this end I would speak to Andy at AP performance he sells a system using paddles an a bespoke cable operation, this has been used extensively on their race cars and came recommended. Then if you do the under dash handbrake mod you have a clutter free tunnel.
Diff ratio, are you going to run 13 wheels ? I found that you were always looking for another gear when you were in sixth, again Nova Racing offer a higher 5th +6th gear ratios to help if this is a problem guess it all depends what you will be doing with it...
It look some time to find my Sierra diff 3.14 but they are out there
Hope this helps, not sure where your based but if you want to drop by for a chat and look at progress on mine let me know.
On the prof shaft I spent a great day removing my diff having found out the only way the prop fits is from the rear before the diff :- lots of swearing........
Paul