typically an aircraft is only certified to carry the same type and model engine in an engine underwing pod as fitted to the aircraft.
The 747-100 was limited to the JT9d because the 5th pod front mount banana link had a smaller diameter pin where it attached to the front spar. The -200 had two holes in the front spar the upper of which had a larger diameter hole for the (larger) pin and a corresponding longer banana link. The rear hardpoints were all the same 8 off 7/16 dia. bolt holes arranged in two groups of four for the rear mounts.
We've carried Jt9d and Rb211h engines on the -200 classics.