It's not backwards. It catches on the woodruff key and goes about 2mm back, then stops.

Also, I can see the inner cam gear right behind the front main seal, and imagine it's running into that. But why won't the cam gear go back farther?

 

Did you change out the oil pump gear? I put a gear in backwards once and tried to tighten the pulley and it pushed on the gear and cracked the retaining plate.

It was my first one and I was not quite sure what went wrong but in hindsight I believe I installed the gear backwards.

 

I did change out my oil pump. Finally figured it out after shedding much anxiety, taking all my pulleys and the front cover off again, and putting the cam gear on over and over, as it came out from the middle gear every time I tried to put the cover back on. Finally twisted the gears just the right way, so when I rotated the front cover to finnagle it on, it slid in.

So when I was trying to put the crank pulley on, the cam gear was dislodged right behind the front main seal and stuck, unable to slide back into the gears. So the pulley was hitting that. After I got the front cover on properly, I did a test fit with the crank pulley before installing all my bolts and it slid right in.