Great project Minkeo
Pleeeese use a manual gearbox, you will die of boredom with an auto.
Remember that the RV8 comes in many capacities and with 94 mm liners and machined down Buick crank you can have 4800cc with the original engine numbers

Lots of help here

http://www.v-8.org.uk/forum/

Mark

EDIT because I did not read the whole post, a 2 litre diff will easily cope with the 3.5 engine, my last Sierra was a 3.5 V8 using a T5 box and stock 2 litre diff.