Towing Capacity 72 dodge dart custom V8

I have 1972 dodge dart custom with a 318. I've changed some things to
it. I bought and engine from a guy that had a 318 in his roadrunner.
It had a 4bbl carb on a dominator intake. He said it was a rebuilt
race block enginge. I had never heard of a 318 race block so i'm
thinking its just a normal block with upgrades. It had a 904 towing
transmission. I didn't know if i should change to a 727 with the new
power it has. If i didn't need to change what would the towing
capacity be? I don't wanna change it. I know a 727 is the cream of the
crop but i don't have all the money in the world and if i don't need
to switch transmissions i don't want to. And i don't really wanna get
a 518 either.