fresh 727 problem

Yes usually an assembly issue... don't feel bad many before you have done it.
Next time around cut the heads off a couple 5/16 bolts and make a couple studs as guides.
Lube the pump oring, sealing rings and case up and the pump will slide easier. Also if you have the band struts together back the band off some so the drum will have room to wiggle.
Measure the old sealing ring with a micrometer against the new sealing ring for width and thickness and if they're the same you'll be ok to reuse one of the old rings as long as it's not stepped on the inside.
Thats if you don't have access to a new sealing ring kit and don't feel like buying a complete seal and gasket set.