Engine options for 75 Valiant

OK, planning out my daily driver Valiant build. Yeah, I don't have the car yet, but whatever. :D

I have owned/worked on/etc GM cars for as long as I have been driving. I know what to look for and what options are out there for a GM engine. I have probably owned 20 Trans Ams now along with a handful of GM trucks.

My only knowledge of Mopar stuff is swapping in a 3.8L in place of the blown 3.3L in a 95 Voyager I owned, along with building a 2.5L turbo for my old LeBaron. Not exactly the knowledge I need for this one.

I was thinking about a late-model Hemi for the car, but unless I can find a wrecked donor car I'm looking at $2000 for a junkyard engine. Ouch.

So what do you think would be a good option for this car? I want something that I can build for good power, get good reliability out of, and hopefully get decent gas mileage out of with an overdrive transmission.