[Will]

you MUST check the valvetrain setup first. There is no hope that the engine will run right and there is the potential for damage if the valvetrain setup isn't right.

I'm going to guess multiple problems here.

First, that is too much cam for stock valvesprings even with stock rockers. With 1.65 rockers I guarantee you are coil binding if the springs weren't changed. This will wipe out your cam & lifters and/or pushrods very quickly, sending metal throughout your engine. Don't drive this car any more 'til you've checked on the valvespring coil bind issue.

Possible that the pushrods are now bent from binding on the heads because the slots weren't elongated. The heads must be removed to elongate the pushrod holes.

Possibly misadjusted lifters.

A stock '77 Q-jet will never work right with that cam without making internal modifications to the idle circuit. If you want to take on the task of learning about Q-jets and how to modify them, get yourself a copy of Cliff Ruggle's Q-jet book. Otherwise, find a Q-jet specialist to set this carb up for you. I'm sure board members can suggest someone good. Or, buy a Holley 750 or 850 mechanical secondaries if you don't care about gas mileage. A Holley won't mount to the stock intake though, so you'll have to change the intake too if you do this.

The 280 magnum is a good sized cam and adding 1.65 rockers effectively makes it bigger. No matter what you do it's not going to have a butter smooth idle, it'll be fairly choppy, but it should be possible to tune the engine to idle without it dieing, though the torque converter will give you problems as mentioned.

Honestly, I'd recommend you make your life a lot easier and get a smaller cam. That low compression 400 will run much stronger with a smaller cam and you won't have to change everything else on the engine and car to match it. The comp XE262 would be a MUCH better match.