Do it, if he's doing a new clutch, and flywheel also and the vibes go away then you'll know. You should be all over this like a College freshman in a 4LOKO warehouse. It looks like a solid bolt on car that will take much abuse.
Forged pistons in the 5.0's went away in 91 or 92 IIRC. Depending on info source.
Get one of those adapters that will go on the rad to pressurize the system. It may not be a HG. Water leak that runs over an oily spot on the motor possibly. I've got one somewhere. I'll see if i can dig it up.