Seems like something with timing is still not quite right. It needs to be set at 16 initial without the vac. advance hooked up and the idle as low as it will possibly go to make sure no mechanical advance is coming in.

The carb could be a little lean for that cam, but shouldn't be that bad. Try backing the screws out a 1/2 turn more each.

The 280 cam shouldn't have a problem idling at 850 rpm. The off idle stumble may or may not be a carb problem. Have to get the timing positively squared away before tweaking the carb. But, most likely with that cam, the squirter nozzle and pump cam is going to have to be changed to make it crisp.