speedo question

My question has nothing to do with the original post
, but it is about speedometer.

I have a 65 Dodge GT convertible that came with a slant 6 and automatic. I swapped out for a V8 and 4 speed from a 65 Barracuda. Also, when I bought the car it had Michelle 205/70 R14.

My question is what gear do I need in the transmission so the speedometer reads correctly.