JP i think using the 5spd prop can be a lot of hassle, and maybe even need to be shortend as 6spd box is longer.. plus if he uses 5spd prop with 6spd rear diff, think you have to change flange or widen out holes.
Best option is, to try get everything from one donor car.. 6spd diff, prop, box, gear linkage, starter. thats route i took anyway. and i highly recomend using the 6spd flywheel, that way you can use an exedy stage 1 clutch, they are not harsh and able for loads more than the 5spd stage one. something like 500 to 550 ftlbs torque whereas a 5spd stage one is not really happy at 400ftlbs
I think you will be alright Mark, think you are still running 2ltr, so you arent going to have 400 ftlbs on a 20g, probably hopefully, max i would think of somewher between 300 to 340 ftlbs, anyway a good mapper can map out some torque to save the box a bit... Just a bit of advice from someone thats been there and done it, think carefully on your next steps mate, there is a certain bhp/ torque figure you can go to, that the car will still be reliable, driveable, useable, and only costing a small fortune... the next step is a big one costing a big fortune, your talking AROUND 4K plus labour for gearbox and clutch, Not worth it in my opinion. If your engine is not forged you be best to stay around 380bhp max