66 dart 904 tranny problems

I'd keep the 904 too for 3 reasons. A converter is hard to find that has the pre 68 trans. input splines and a big center button (register) that all 68 and newer engines use, and 2 the 727 is a pretty tight fit in a early A-body so it's harder to work around, and 3 the 904 is plenty tough in stock form up to 425-450 hp. Even then you can use a 904 with more horse power if beefed up. It's your choice on keeping the original engine or putting in a 360. The 360 will make a lot more power especially if built up but the 273 is a fantastic little engine. It's a matter of preference.

If you do put in a 360 the torque converter for the 273 will bolt up but the center button is smaller on a 273 so it'll let the converter wobble and break the input shaft and trash the front pump. You have to get an adapter that slips on the converter register to use it with a 68 or newer engine.