I've got a 68   Javlin,6-banger 3-on a Tree been sittin out 20+ years. Want to fix it right the first time.
My first thought is to get a donar Mustang GT?
My question; Has Anybody been there done that? I want a cruiser with pep, not a dragster. After all I live in Texas  and its 100 miles to anywhere.  

If you want to keep it as a six you can add a 4.0 liter Jeep head and manifolds or you and swap in a complete 4.0 liter from a jeep with fuel injection.  There are lots of website posts on swapping in a 4.0 liter. The cylinder head swap has been discussed alot too.   Do a google search.

If you want an AMC V8, you'll need a V8 Javelin/AMX crossmember.  You can get a used V8 wiring harness, or modify your harness and or have a new one made.  The little T-14 transmission would probably be OK behind a 290 or a mild 343 but won't hold up behind a 390 or 401.  Same for the rear end.  You would need to get an AMC 20 rear out of a similar car or go with a Ford 9-inch or Mopar 8 3/4 rear unit.    And no, I have not yet done this swap but may do a similar swap on a Hornet in the future.  

Might this be the Javelin in Weatherford? Anyway... I know of a pair of 68/69 Rebel SST's you can get cheap, both have 390's. But a 5.0 swap... no telling how much fabrication you'll have to do.. wiring, computer, A/C, radiator spacer, custom driveshaft....