All layers of paint need to come off to ensure the "bad" paint is totally gone. Back to bare metal is the only way to be sure.
I am not sure what you can do but you need to make sure the rust is not working away under that paint.
Maybe you are better off to sand the cracks and seal with aerosol...
I do not think that will work out as you are likely to breath to much of the oil fumes back in to the carb and therefore polluting the intake charge coming in to the cylinders. It will ventilate the crankcase but I believe you will consume oil and foul the spark plugs.
Doing what you are...
So by the sounds of it it will still be externally balanced. That can make it tricky. It is hard to balls up an internal balance, but due to the counterweights being so far off centre with external balance a little too much or not enough can cause a problem.
You will be OK with those Edelbrock heads they are no worse than other named brands. I have come across faults on two other reputable brands - nothing that could not be fixed but a big NO to run out of the box.
The best part about all those heads is they use good quality components - valves ...
Yes they are a shop that specialises in the A833. They know the trans inside out.
I just typed brewers a833 and went straight to their website.
They sell a little install tool for that inner seal also if that helps.