Not as Hipo as it sounds. What's up?

Sounds like a textbook example of a "miss-matched combo". Small port intake, big port heads, really low compression, big cam, small exhaust system, tall gears, low stall converter, etc...

The heads have been "milled to fit the 273"?!? They are a bolt-on. I hope that means they have been milled A LOT to raise compression, like .060-.080. If you didn't have to order custom shorter pushrods, they haven't been milled enough. I am guessing you have about 7:1 compression.

I agree with just installing a 360. Even with the milled heads, you probably won't have too much compression, but 75 heads aren't anything special so look for a 360 with heads on it. A late model 5.9 Magnum truck engine would be a good starting point.

Yes you could use that cam in any LA engine, but not knowing what it is... Plenty of nice rollers out there for the Magnum.

Smash the factory exhaust manifolds with a hammer so you won't be tempted to use them. Cut the single exhaust into tiny pieces for the same reason.

A 360 will bolt to the current transmission. You'll need a counter weighted flex plate. Converter selection is limited for the earlier transmissions. Just how long do you expect a 47 year old low performance transmission to last behind a high performance engine?

The 7 1/4 will break as soon as your engine is capable of generating torque. Are you still running 9" drums ans single circuit master cylinder?