I used the Manton 3 series 5/16x.120 wall with springs that where over 700# over the nose with no problems spinning the motor to 8000 and these where recommended by Terry Manton.

70 duster full chassis super pro 416 CNC Indybrock heads 727 w/brake

best so far 1.212 60 6.219 in 1/8 at 110.88 9.768 at 137.81 1/4