273 or 318 or both help
I’d run with the 318. But a junk yard Magnum would be easier and cheaper as well as stronger. You’ll need to get an intake, a chink knock off is fine here and a distributor. I don’t know how well the exhaust manifolds or headers line up. I remember an issue with some people with some headers.
Grab the converter with the Magnum if you can. It’ll require a simple mod to the flex plate for use.
I grabbed a 5.9 out of donor Durango and added the above parts. RPM intake, 750 carb, electronic distributor & Headers were Hooker Super Comps for a B or E body. This is in a ‘79 B body car. It’s an excellent driver.