The difficulty of the conversion depends a lot on how correct you want it to be when you're done.
The pedals are the biggest obstacle because there aren't a lot of them around. The floor hump is out of reproduction so if it has to appear concour correct, that's another obstacle. I think Brewer's can supply most of the linkage doodads and they sell restored pedal sets but rarely get C-body ones.
If it just has to work properly and not look like a hack job I think the worst part will be finding a pedal set. Some fab skills and a repro B-body hump plus a new carpet solves the interior and the rest of it just takes $$.
If Passon Performance ever gets their production problems sorted out on their new bolt in 5spd OD, you would have a dream 3 pedal C-body.
Kevin