Yep, a 750 is too big for a 350 without lots of mods. Use this formula to calculate a good size
Size = 405 CFM !!!!!
Of course this isn't real life nor realistic, but it does show that you are way over carbed. Your almost stock 350 just doesn't have enough in the way of being a vacuum pump to make this carb efficient in allowing enough air into it and atomizing the fuel/air mixture well enough. Of course, good heads, a 'big' cam, a few more cubic inches could make this carb work - at higher rpm's, that is
**CID = Cubic inc displacement
RPM = Revs per minute
3456 = a standard constant
VE = Volumetric Efficiancy (.80 or 80 percent is very good for a stock or mildly modified engine)
The biggest carb I would use on this engine would be a 600cfm and when modified, a 650cfm might work well. If you need a perspective, my old F350 with a 460 engine had as OEM, ~a 500cfm Holley. I tried a 780 Holley, but it ran best with a 600 cfm unit, either a Holley 0-1850 or later, an Edelbrock.
Exhaust gaskets wont make an engine run that badly, just sound crappy. Of course, ignition systems if set up "off" will and electric fuel pumps set at too high or low pressure can cause big problems. One question - GM's use full manifold vacuum - do you have it plumbed there? Ported vacuum will not allow enough initial advance (Ford's are opposite and use ported)