Pop pop on exhaust side

In reverse order of importance IMO
I don't know about the later heads but with the earlier 273/318/340, no exhaust gasket is needed. Flat to flat.
On the exhaust itself or with headers, I prefer thin so the clamping is more even and less likely to distort the flanges, BUT many headers in particular are made with a rough raised bead - and those need a thick gasket.

Exhaust size. Big tail pipes, big tubing, short exhausts, allow air up into the system at low rpm and decel. It used to be pretty common to hear pops in the exhaust of sports cars as they decelerated during races. Excess fuel, no vacuum advance, short fairly open exhausts all contributed.

Initial timing seems to be ballpark correct - as always depending on rpm. I don't think that's the cause. With no vacuum advance, the mechanical can come in quick. Since we don't know the specific conditions when the popping occurs, hard to comment further.

Fuel. This you may be able to clean up. If the other posts about the 703 are correct, the idle should have decent vacuum for a cam with that lift and duration. I suggest a thorough mapping out of the carb. Start at the begining.
Fuel levels.
Then take it off and check the primary transfer slot exposure.
Note how many turns of the idle speed screw to get to that exposure. On an older Holley. the primaries should have a .020 to .040".
With strong idle vacuum, slightly less exposure will be needed depending on the idle feed restriction and idle air bleed Holley used.
If you have the drill bits, measure the idle feed restriction and air bleeds.
Count how many turns out for each of the idle mix screws.

My thinking is that the fuel level may be a bit high in one of the bowls, or the transfer slot exposure is a little bit too much, and as a result there is excess fuel once the engine is cruising above idle speed.

Leaning out the off idle and increase the idle mix to get the idle where it is now. That would be the approach if the problem is excess fuel. A little higher in the throttle opening and it might be necessary to mess with the air bleeds but thats getting deeper than may be neccessary.