Thanks in advance for any advice you guys can give.

I am building a bored 454 (468) for a 1969 El Camino. The block, cam and heads were acquired separately over time. I am having a hard time getting the distributor set right. As far as I know all Chevy’s, small or big block, the #1 cylinder is the first one on the drivers side and the firing order is 18436572. I have repeatedly set the #1 (driver side front) at TDC but all I get is backfire through the carb and/or exhaust. On further inspection I found that the fist plug on the passenger side is closer to the front of the block, than the one on driver’s side. Could the first plug on the passenger side be the #1 cylinder? Is this a marine block or something funky? If this is not the case does anyone have any advice.