I've changed barrels on old milsurps and LoonWulf is right--a sharp blow helps remove a stuck barrel better than simply more torque. The tapping around the joint can also help removal as it breaks the mechanical lock between parts. There is a reason that impact wrenches make removing stuck tire lugs much easier than a hand wrench. Mildly Heating and cooling the junction can also aid in removal as application of something like Kroil to the junction. I don't believe that Ruger uses any type of locking compound on the barrel nut/receiver interface but might want to check first as it will lead to frustration in barrel removal. You will also want a torque wrench for reinstall and know what torque specification is for these rifles (a lot of Autoparts stores will lend you the tools with certain conditions). AR's are pretty forgiving in torque specification with a wide range but you might want to check with Ruger if they have specific torque recommendations for your rifle.
One key thing, make sure that you have a barrel vise and try to figure out how to protect the finish of the barrel and receiver when removing. There are a bunch of suggestions around that can work--eg masking tape for some applications, lead shims, etc. A pitcher's bag of rosin contains more than enough rosin (at local sporting stores) to remove and install a bunch of barrels and is far cheaper than Brownells. The rosin is a bit messy but is non-marring and easy to remove with mineral spirits and it helps the barrel stick instead of twist in your barrel vise.
Others will have better advice on how to fix the upper receiver in place as I am unsure how much the RPS receiver dimensions are similar to the AR-15 or AR-10 receivers. If you are making a switch barrel system, then you will need to buy the proper tools to do it right because it would be a regular process for you. Brownells, in particular, has gunsmiths on staff and will answer questions about what you might need to do this on a regular basis.
