Well, it's your call, but if it were me, I would want to know for sure what was causing the problem, then make the call based on price and time.

If you are positive it's the head gasket, then compare costs in both time and money for each option. Go with the cheaper of the two.

If you can get an engine with better miles for cheaper or even money, then do the swap.

Or you could go crazy and swap in a 5.7 HEMI

If you aren't sure about the head gasket, it's worth it to do a full diagnosis.. Compression test, coolant leak test, etc. There are a lot of things that can cause a rough running engine.

 

Here is the compression info. Any ideas? REAR = Firewall
FRONT = Headlights/Grille

 

Whoa. That's bad...... Pull the heads, and see what ya got?

 

Yeah, that's no bueno.. Pull valve covers and if the valvetrain looks ok, I would say that confirms bad head gaskets or toasted valves

 

The reason I recommend pulling valve covers is to check the rockers.. The 3.7 and 4.7 for some reason like to toss them. If the intake valves on those two cyls. aren't opening, you could get a no compression reading. Here is a vid: