There is a gentleman on this forum Pre '68 Dave, that has a web site that explains the forward position.
[LINK]The existing bell crank and linkage that came with my truck, was from a car, it did not work well and I was afraid it might stuck. I bought original 216 linkage and bell crank from a fellow “bolter” which necessitated the move to the forward position. The throttle is as smooth as silk now.
Your right in the fact that the rear hole has a taller boss that does not penetrate the water jacket, so no sealant is necessary.
