We have a '79 that was stored since 1982. Low mileage, but carb all siezed. Tho I did get car running. In August bought a '79 that had new factory motor in it with 25,000 miles on same. Body poor. The person we bought it from drove the car here and we ran engine a few times prior to pulling it. All components looked low mileage, clean etc.
I pulled motor from original car and did the swap. Donor motor sat about one month prior to being installed as I had to sand blast and redo engine compartment . Once installed "new" motor almost impossible to start.. But ran great month before. Now when started runs with no power only while choke is on, once chokes kicks out car stops. Power when running under choke barely moves car in first gear.
I have changed timing and gotten a bit more power but not enough to make a real change. Installed new plugs, points, rotor cap, wires. Checked coils...ok then swapped from donor car as they were making engine run month before..no change. Changed fuel filter, installed new rubber fuel hoses. Fuel pressure semed ok. But since I had pump from donor car, swapped that too!.. Checked return line, can blow through and feel a bit of resistance...This motor when running smokes not at all which was why I did swap in first place....Any ideas greatly appreciated...ed

 

sounds like what happened to me, the accelerator pump in the carb was totaly blocked up, so i could get the cer started but but was lucky to hit 20kph when i drove it.
try rebuilding the carb. i had the carb changed because my pump was unblockable and now its perfect.

 

I've had some weird things happen when swapping engines in as well...
That was a year ago and I'm still trying to sort it all out so persistance is the only advice I can give you...The car sat for 10 years think maybe the exhaust is blocked with a squirrel nest?