360 converter

It's not going to be that simple. Early 904's used an 18 spline converter and the 273 had a smaller 1.55" pilot hub going into the crank. Any converter made for a 904 that was behind a 360 will be a 27 spline so it won't work with your old style transmission. The only 2 solutions I know of is to swap the whole trans. to the newer 27 spline because no mfg. makes a 18 spline converter with the larger 1.81" pilot hub. A friend of mine just went through the same thing on a 65 Dart he bought. He called every converter mfg. he could find and nobody made one that would work.

The only other solution is to disassemble the trans and put a newer 27 spline input shaft in it so you can use a newer converter with the larger hub. Any way you go it's not as easy as just a converter swap.