FWD BLOCK ABSOLUTELY DOES NOT FIT THE RWD SR TRANNY. I'VE TRIED IT MYSELF.
The fwd head will however fit on the rwd block. i wanted to use an s15 6speed so i had to get a rwd block.
if you want to use a z tranny, [or possibly a ka24 tranny, since they've been adapted to L6's without much fuss, but dont take my word on that, i havent done that conversion] then you could use your fwd sr20de.
to my knowledge, a rwd intake mani would bolt up as long as its around the same year, because the changed them at some point[high port/lowport]
there may be other issues that arise that you'll have to solve on your own because for the most part, you'll be in uncharted territory. everyone else just uses rwd, because it doesn't seem worth the fuss to mess with a fwd de.