I currently have this cylinder head in mine
http://www.airsoftparts.ca/store2/in...oducts_id=1819 It's the only other one I've tried and it works fine, this leads me to believe that all regular V3 cylinder heads will work. I did have to shave down the second tooth on the piston because of the sorbo on the new cylinder head but that's not unique to this gearbox. The Mask piston head seemed too tight in the RS cylinder so I'm using the stock one, It looks of better quality anyways.
The cylinder head and the spring guide
http://www.airsoftparts.ca/store2/in...oducts_id=2081 were the only things I have changed out in the gearbox. I find everything else to be well made with a good air seal.
You're using the stock spring guide? If you can't get any extra room via the spring guide I recommend you look into a different spring or cutting your 130 down. I imagine there is some info already out there on different brands and their lengths.
There's another guy on here I think it's Amos who seems to be familiar with the Type 56 gearbox.