C6 would be a good swap.
The FE C6 should bolt right up to the 352.
Early "pull type" starter won't work, need the later type.
The cross member (cutting and welding) would need modifications.
Possible driveshaft length and slip yoke different as well.
Cooler line differences too.
Probably need a later 352/390 flex plate as well.
I put a C6 into a 63 Galaxie to replace the cast iron Cruise-O-Matic.
Had to mod the cross member, cooler lines and needed a late type starter and flex plate.
Also need the dipstick and tube for the C6.
Vac line for the modulator.
In this my case the drive shaft went right in, but I think the 2 spd drive shaft might not work.
I don't recall any issues with the speedo cable.