having hard time with the thumper cam

[69heavychevy]

i was wondering if anyone here was running a thumper cam, #12-600-4 to be exact. i just got thorugh installing one in my truck but having trouble with the timing. i seem to have to retarded quite a bit to get it to even run right. if anyone is running this came what do they have their timing set to. im also open to any suggestions. oh yea i have a performer rpm, edelbrock 600cfm, headers, and 993 heads. thanks in advanced!