Need help determining proper speedo and drive gear

[Nbf77400]

Hello all, I recently purchased a 1972 C20 that previously had a lot of backwoods engineering done to it. I'm trying to slowly bring the truck back to being functional and in proper working order. Long story short previous owners did a trans swap to a 700 R4 and never changed the speedo gear and I have no way to be sure they did the drive gear either. How do I determine the proper gear that needs to be swapped in so my speedo doesn't read 100 when I'm crushing at 70.