models and materials quandaries
Which Forespar model is being discussed? The one with the triangular base that takes three screws or the newer "OEM style" with the rectangular base and no screws?
The later one uses a buttress thread and can *only* mate up to the matching threaded thruhull piece.
As to using a bronze thruhull with the older Forespar seacock, I would just "cut to the chase" and dime up the forespar folks and ask them. It's their product, based on their engineering and R&D. They will advise you.
I have not phoned Forespar, but have asked for technical advise from several other marine product companies and *always* received helpful and forthright assistance. (Harken is tops in this regard, FWIW.)
We all have some annecdotal "field testing" to relate, and will be glad to advise as best we can... but going upstream to the source is always your best move, if you can do so.
Please keep us posted,
Loren