I am having some ammunition made up for my pre-64 mod 70 375 &458 and a custom springfield 30-06, using the GS fn bullets. Am I likely to have to have any gunsmithing done to allow correct feeding from the magazine due to the shape of the bullet?

On a post-64, probably nothing. On a pre-64 there is probably a 50-50 chance of modifications being needed. On the Springfield, I assume you are looking at HV not FN bullets. If so, no modifications are likely to be needed.

No; I was planning on replacing the barnes solids with the GS FN solids

A Springfield 30-06 with GS Custom FN solids? Just making sure I understand. That will probably need feeding work.