Chuck-ia,
The later Gillespie's made in the Mills River NC shop of Mathew Gillespie son Philip, were almost all percussion with a sprinkling of die hard flints built for shooters that still insisted that if God had intended us to shoot percussions he would not have made rocks!
Mathew's other sons Harvey, James and John made signed percussion rifles, as did their sons. Probably many of the rifles made by Mathew's two brothers William and Robert Harvey made unsigned percussion rifles.
So, if (oh it hurts me to say this
you want to build a percussion Gillespie, and feel like it would be period correct I would not hesitate to do it.
One thing I will mention is most of the percussion Gillespie's that I have seen have had straight barrels but I, like you, much prefer the swamped barrels.
Dennis