I think the exhaust issue has been explored before.
What I do not understand is the fume issue from the breach. The bore evac is not working? I have loaded/ fired M 48’s, M 60’s, and M1A1’s. they all work just OK. Is the problem the fumes from the spent shell’s? There is no practical way if the turret hatches are open to deal with the fumes… You could not have a positive or negative pressure system. You could pressurize the breach to blow out more of the fumes but the shell casing will end up smoking on the floor.