REAR shock suggestion?

[turbotruck]

68 c10 355 turbo sbc
stock suspension
stock shocks.
last year with the less powerfull motor i ran 10.09@131
i spent all day today trying to get it to hook with the new motor. best i could get was 11.21@130
it hits off the line ok. can cut a 1.5 60 foot. but then it spins at the top of first gear, pedal it, shift to second, power comes in. spins. pedal it shift to 3rd. power comes in and it spins.

i m thinking my 15 year old monroe shocks are no longer cutting the mustard

what is a budget friendly rear schock for the 67-72? its lowered 4 in in back. 3 in drop springs and a 1' block. has the cpp shock relocate brackets installed