That was my first initial thought when I was lying under the truck fiddling with the trans mount. I got up and looked in the engine bay. The truck engine mounts won’t go on the block other than how I have it AND the distributor cap is as close to the firewall as it can go.
The engine is in the right spot.
The engine mounting brackets go on the back side of the block ears. It is possible on earlier blocks to move them to the front of the ears, that is how I moved the engine rearward in my Duster, and with a little work you can do it on the later blocks as well. Post up some pics of your mounting brackets.
A long tailshaft is a long tailshaft, the trans mount bolt holes are all in the same place. Maybe post up some pics of the trans, mount and crossmember. Some trans mounts have elongated holes where they bolt to the tailshaft, I've seen some that don't.
I don't think distributor to firewall clearance is a good indicator as if you raise the back the clearance will increase. So exactly where the height of the tailshaft is will affect the clearance at the firewall.
