I'm NOT a transmission guy, in which case anything I say is really only a muse...
If trying to spin your axles a bit doesn't introduce some slack (don't try and put a lot of force) then it's got to be the transmission or the clutch (though how the clutch could be jamming things I do not know- assuming that the transmission hooked up to the block just fine- NOTE: I had a case where the thin metal plate that foes between the trans and the block had an ear (slips over the dowel) fold over; I was fortunate in that I caught this before I started to torque things down; this would have ended up causing things to be a bit twisted- no idea if it would have resulted in a binding or not, but certainly not right).
If a more knowledgeable person comes along they could say whether it would be meaningful or not to pop off that end pan to expose the 5th gear assembly.
Are you sure the clutch fork and throwout bearing are OK? Not thinking that even if messed up that they could cause things to lock up like this (unless something jamming into the fingers on the pressure plate's fingers).
OK... one more thing... and, again, not sure how it would cause the trans to stay locked up, but the starter, might pull it (can look inside a bit). Things locked tight don't provide enough wiggle to allow one to disengage gears.