Score! What did I do?

Well, I have been inducted into the world of early A-body Mopars. I recently snagged a '64 Valiant 200 4-dr.(ehm) Very decent white paint, Red interior with a factory V-8. It has the pushbutton auto (904, I guess) and I like the stance it has. VERY little, tiny rust spots in the trunk.
I got it for less than $1800.....so I don't think I got burned.
BUT, here's the deal:
The previous owner told me that the engine & tranny had been rebuilt, and the engine is -clean-....so I sort of believed him. After pulling the valve cover...I have my doubts. They put a later 318 heads and intake on the engine...I don't know if that's good or bad. My worry is that they reused the stock *mechanical* cam, and just used *hydraulic* parts everywhere else.
The heads are bad enough that I'm not starting it again until I pull the heads and clean everything up.
Other than that, the car is nice......I bought it as an "easy" ,oh well, project to have until I get my Orange Dart fixed. I like the little 4-dr. strangly, and although I miss my Swinger, I may keep the Plymouth too if I can fix it relativly easy.

That brings up a second option though......I have a 3/4 built 318 Poly motor in the shop (W/factory 2x4 intake).............could/should I simply finish that and stuff it in the Valiant, or should I fix the 273?

I'm all ears to any suggestions and comments...