I have seen the bike that Quadzilla vs. purchased...it is definitely factory original...meaning that have been no cuts or re-welds since it left the factory. As far as if it is a 24, 26, or 24R/26F...not sure. The rear triangle sounds and looks like a 24...but why would a shop well versed in BMX bikes in the 1970's & 1980's (Sundance...Jim M. specifically) have placed 26 wheels and fork on a 24 frame? I would have to think they would not have done that...but who knows?
One way that I can think of to figure out if it is a 24R/26F is to measure the head tube and seat tube angles with the wheels in 26R/26F and 24R/26F configurations. If it is not a 24R/26F...when you have it in the 24R/26F configuration the head tube angle will be too relaxed (you will have to find out what the head tube angle was supposed to be).
As for measuring the tubes:
Top Tubes: Center (head tube) to Center (seat tube) along the center of the top tube (not the top of the head tube and seat tube because the head and seat tube angles can throw the measurements off).
Seat Tubes: Top (top tube) to Center (bottom bracket) along the center of the seat tube.
Hopefully measuring the head tube angle will help to determine what GT was doing with this frame!?!?
If it's not a 24R/26F maybe it was a try at a XL or XXL 24?
