Timing Gear Drive
I know its frustrating to wait for an answer. I can't give you one, sorry. On my 392(which interchanges with sbm timing parts) I just tapped the crank gear on all the way aftre deburring the key, didn't think to check the clearance on the back, but since you brought it up i will. But the gear teeth line up nice. If its seated and the lined up, I dunno what more you could ask for. I wonder if its possible you have the gear on backwards or a bur on the snout? No offense, I had to trial and error to find the right orientation.
Maybe this weekend i can finally get around to finishing up my drive. I had to pull the shortblock apart when i spotted a tiny chip of metal resting on the girdle. Did I not clean the block well enough or was it some lost chip that got knocked off the bench or something? Dunno but can't take chances.
I had to have the recess on the plate deepend because it was just not made to the right spec. The one good thing about that is that it allowed the machinist to measure the cam washer and machine to a specified clearance rather than going with whatever it might have been had it been machined right in the first place. I realize that while the timing parts are supposed to interchange, this set was labeled for a 392 so there might be some differences with the 318 so take whatever i am learning with a grain of salt.
I'll measure the backside clearance if any and post some pics including side view. Hope you get yours figured out.