Need driveshaft for 70 Dart Swinger, smallblock 904 8 3/4 to handle 500 HP, Best place to buy

70 Dart Swinger, 2850 lbs
SB 408, Bullet roller 264/268 @50 .636 SSDS stage 2 Edelbrock heads, 1 7/8 Headmans, 1050 dominator by Dom, 9.867 @ 133