We had the same problem.
It's a timing issue.
She will run great without a load.
Put a load on her and she'd stahl out.
Put her in the water
Get up to 1200-1300 rpm
Loosen distributer clamp & slowly rotate one direction until it runs worse.
Then rotate back the other direction.
You'll hear the engine picking up speed and be able to set the timing dead nuts while under load.
Much more acurate than sitting on the driveway.
After timing is set, back it down to idle and reset the low idle throttle linkage to 750 rpm.
That should do it.
http://www.thecarbshop.com/
Yep, that's where we took Hanna's carb.
Damn good shop and well worth the big bucks you'll pay there.
But It'll get done right and turn around time is 24 hrs.