I am having issues with my right side C body big block exhaust manifold gasket blowing out every 500 or so miles. I replace the Fel Pro gaskets and it is ok for a while, but will gradually blow out. I clean the surfaces on the head and manifold each time to make sure everything is as well prepped as possible. I have done this three times.

There may be a slight crack in the manifold, or it may not be quite straight, but I plan to switch over to some headers when I can find some at some point with the right price. In the meantime, has anyone used any kind of RTV on the head side, or manifold side of the gasket to help sealing out? Is double gasketing worth a shot?

I am not inclined to remove and resurface the manifold.

Make sure that you are using gaskets that have the port that matches the port in the head/manifold...no larger. Some gaskets for headers have port openings that are both larger, and corners that are more square (less radius) and will leak shortly after installation. I have seen this more than once.

Use RTV and NO gasket, make shur everything is clean, let it set up completly. So long as you dont hav lots of exhaust back pressure it will prob stay put.
I sealed an 1/8 header gap that way, quit blowing out once I quit using gaskets

Use high temp copper rtv on the gasket, or leave the gasket out and seal it with just the rtv.

both my mannys have copper rtv and no gaskets

used brake clean to wipe the heads and manny down,then used copper rtv on them,let cure over night and no leaks so far.

done it on warped header flanges as well.

I use copper rtv on both sides as well, doesn't seem to matter what type of gasket, it sealed up headers that had always been prone to leaking after a season of driving.

The important thing here is that MOPAR never used gaskets initially when they built these engines and cars. Just metal to metal. As JohnRR stated, you have warped manifolds. I had the same issue with my now sold C Body and after having the surface of the manifolds trued up by my shop, they bolted right up and never a leak or blow out. Get rid of the gaskets...

One word,REMFLEX.Best gaskets out there.1/8" thick,graphite.Follow directions,good costamer service well worth the money.