There's no easy fix to replace the rockers: find something, probably from a large engine.
I wonder what the later GMC 305 V6 rockers look like? Too short.
Ford 534, etc. also possible.
The 2 factors that have to be very close:
1. length almost the same (pushrod hole center to stem tip)
2. ratio > 1.5, which means the shaft has to move toward the pushrods
For inserting a port, a large-radius piece of Schedule 40 weld-ell in 2" ID allows you to make the insert at an angle and have either a side-draft or down-draft port without fabricating the curve itself.
However: Toyota F (Land Cruiser) engine has some commonality with GMC, and these parts are easy to find. However: there are several chamber types as to valve angles, etc.
