honestly you kind of picked the wrong marker to go milsim, iv never heard of a stock for a fish, best thing there is to get a butt plate for your tank, unless you already are running a remote line, feedneck is the right term your looking for but im not quite sure what you want to do there
as for the barrels, well they should fit if they have the correct threds. but dont quote me on that, i could be wrong, if you want a marker that is a bit better than your fish and can be a very nice milsim, look into the sp1
good luck and if you can describe what you are wanting to do for the feed better i may be able to help you there