Think about it....

Your current stroke is 3.48 (standard 350 chev stroke)
You want to change the crankshaft to a 3.75 stroke

This means that the piston will travel 3.75 - 3.48 = 0.270
further; half of it up and half of it down. SO 0.270 divided
by 2 equals 0.135 which is how much further up your existing
piston would travel.....you aint got room at the top for this!!!

So...you must either get a shorter rod (mucho expensive) or
buy new pistons for the stroker.....buy a stroker kit which
includes the crankshaft and the pistons matched for the application.
You'll be better off in the long run than trying to piece together
a mismatch of parts.

mike in tucson