Hello all,
I'm not new to building and restoration ( since 1987 ) but after the hundreds I've worked on I am now the owner of a 1956 Oldsmobile Holiday 88 hardtop 4-door sedan. It DID NOT come with a motor or transmission and I'm interested in what would fit within the frame rails without major modification. I'm going to build a modern mild custom for daily driver usage, and after building a '49 and '51 chopped Mercury and regularly retrofitting C4 suspension in everything that has 4 tires I'm not interested in building a new frame or any other extensive modifications.
I would like input from experienced owners on keeping it as simple as possible.

 

I don't know what "modern mild custom" entails, but you're going to be limited to first gen drivetrain for direct drop in (303-394/slimjim trans). Ross Racing used to be the go to for 1st gen Olds performance, but I've heard he's gotten flaky these days. You could front clip it with a G body or S-10 to get 2nd gen interchangeability by using 307 frame pads, then making a crossmember.

 

My 56 came with a 455 TH400. I wound up changing the motor mounts from a Tornado style to a more common type mount. Easy fit, easy to make motor mounts, and easy to make a drop out trans mount. Can't even call it a crossmember, as it's so small!