Well lastnight I pulled the bad heater core out of my 68 Barracuda with AC.I only removed 1/2 the box(snap clips holding together)I basically did the same as you,ve wrote,but there was one last bolt holding a clamp at the firewall holding the 2 tubes together.:angry7:Talk about being a contorsionist.LOL.Anyway I got it out and am picking up a new one in 2 hrs(shocking to get one so fast here in Ontario,Canada).The PO must,ve been putting eggs in the rad to try and stop the leak,maybe just hard water?Pics for your viewing pleasure LOL..
That heater core looks pretty bad! My heater core just had a very small leak, just enough to make a stain.
As far as the clamp on the tubes, the one with the bonding wire, that should come out with the box far enough to remove it easily, but...
When I removed mine to rebuild the whole assembly, I tried to get both boxes out together, after I realized that it wasn't going to come out, I split the 2 halves while it was dangling behind the dash and it came out easily.
When going back in, I fully installed the A/C box, and then installed the heater box afterwards. It went pretty smooth that way.
Looking at your pics, it looks like you just removed half of the heater box. If that is the case, I can see where it would have been difficult. Removing the whole heater box and then splitting it in half on the bench would be the easiest way to go.