I have not investigated the OEM rocksliders if they can be converted to air tanks. The drain holes look like they can be welded over or closed with screws. your two compressors work in parallel right? how strong are they compared to the ARB dual compressor? How well do they hold against high pressure cleaning?
So far so good, and have high pressure cleaned several times.
I had the same set up on my old Land Rover when the CMPK 12 first came out in 2007 to when I sold the Land Rover to my nephew in 2020, and it had a hard life and never missed a beat.
My nephew gave it an absolute flogging for the next couple of years lots of beach driving, and the compressor and L/R never missed a beat.
He did over 60,000km in that time never touched it besides oil and filters, and sold it for nearly 3 times the amount I sold it to him for.
But he now loves four wheel driving, and comes on the odd trip with me in his modern 4x4.
I made an air tank out of pvc steam pipe 750psi rated for the L/R so I figured the 120 psi the ARB delivers would be fine, and it was, you can make the pvc any diameter or length you want the fittings are available and very cheap.
I wrapped 10 rolls of insulation tape around the pvc pipe for stone protection so I learnt what it would be like to work in a low skill job that day.
The 2 single compressors are stronger than the ARB dual compressor as they don't have a cooling fan as a fail point, the fan is not water or dirt / mud proof.