I've had the same problem. Seems the manufacturers make different diameter holes and tubes.
I don't know a lot about anything, but I know a little about everything.
Built a 1967 Shelby GT-350 Convertible Replica, a 67 Vert resto-mod and also built a 427 FE 67 Shelby GT-500R (R Stands for Replica). 427 Side Oiler, toploader 4spd, 4 wheel discs, 9" traction loc and a perfect body. What more could you ask for? Now I car hunt and horse trade and I am currently looking for the next project! Always seeking that elusive "reasonable" Shelby.