OK, I just gotta know why?
In 54 years of hotrodding, never once did I swap to a slanty, lol.
Altho, I did rebuild a few.
If it's for fuel economy, that's almost a myth.
I'm pretty sure that I can get better fuel economy out of a hi-compression, modest cam, overdriven M/T 360, than any factory non-overdrive 225slanty ever made; and I'll give you the benefit of choosing any Mopar trans and any Mopar rear gear.
Your 318, properly rebuilt, can do even better.
If it's cause the 318 is wore out, and you just happen to have a slanty with a transmission, well that's another matter.