ls1 swap clearance problems help

[1moretoy]

i have a 70 short step and trying to swap in a 01 ls1 f body motor using s&p motor plates and the stock sbc rubber mounts,and im having some clearance problems. first, the point on the s&p plate hits the ac comp brkt. so i had to take that off just so i could mount the plate to the block. second, the oil pan hits the crossmember and i only need to move the motor back about an 1" to 1.5" to clear it. can i unbolt the factory perch's wich are mounted in the front holes, move them back and drill new holes?any issues doing that, if i do that will my nice new thorley headders still work? thanks in advance.