Every BB and 426 Street Hemi car I've raced with 727 and 833 NP 4 speed liked the 3.5 inch collector extensions up
I would start with the 18 inch long extensions usually and never trim them any shorter shruggy
I did try a set of 3.5x 12 inch long extension on my old M.W. stocker and I can't remember now if they were faster or slower than the 18 inch ones were: confused:

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)