Backfiring 70 C-20

[CRX89]

Hi im new the forum. I got a 70 C-20 350 4bbl th400. Anyways I tried to tune her up because she was backfirin so bad it barely drove. It would back fire when it shifted. I got it to where it only backfires when the throttle is held between 3-4k rpm. I adjusted the dwell but the thing is i cant find the timing mark on the thing with the timing light. I marked with white paint. Could my timing be so far off? Could it have jumped a tooth on the distributor? Also how hard is it to swap to from points to hei. Im too lazy to go my parents to look at my camaro to compare. TIA I hate to dump this truck that is getting somewhat desireable

[4onFloor]

It's very possible your timing chain could be going out. If it is, it problaby skipped a couple of teeth.

[CRX89]

thanks for the reply turns out the timing was to advanced on the distributor.