Is your xr3i running mfi currently or efi? Personally if I was building from scratch I would always go for an efi setup as mfi, IMO, can be troublesome! Circa 250bhp can be achieved on mfi, or even more depending on how deep your pockets are.

As for the cranks I'm pretty sure they are the same but be careful with sumps as s2 sumps have a hole in for the oil return from the turbo but s1 ones don't.

If you already have the piper cam in a decent head then I would be inclinded to use it. Some people don't seem to have problems but others do. I run a newman cam with solid lifters in my track car which seems to work well and doesn't cost the earth.