Welcome!
Your boy sure looks excited about it.
Should be able to determine the gearbox at a glance....
A T-18 will sweep up at the rear and have a number starting with (13-01) cast into the driver's side.
A NP 435 will be square(ish) at the bottom and has an aluminum top cover.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.