What is the difference between the SRT-4 engineand 420a other than being a 2.4L and the head is reversed?

 

Well, there's two out of the way. There's also low compression, turbo, motor mounts, tranny, ECU, wiring (engine and cabin), driveshafts, just to name a few.

 

So if I wanted to build my engine up for a turbo, it wouldn't be as easy as just using the block/bottom end of an SRT?

 

You motor is fine. It's not the strength the your motor's block or head, it's the strength of the internals. The key word is "build". When you say build, I take is as you wanting to have your motor disassembled, cleaned, rehoned, edged, etc. with forged internals installed. The tranny is fine. There's a lot of turbo'd Avenger/Eclipse/Talons that have done the same.
Just about any stock motor can be rebuilt to withstand boost pressures.