I used to build these things for a living back many years ago for Briley then Other Stuff Gunsmithing.. I may be able to help you
first, if you're going to shoot 38 super you'll want a fully supported ramped barrel. Though not a requirement, you won't get the full potental of a super without the supported chamber. Suppers when reloaded hot can blow the bottom of the case...not a good situation. If all you'll be using is factor or factory spec ammunition, you'll be ok.
Yes, the lower is the same for a 45 as it is for a super. The differnce being the ejector. You'll need a super specific slide, ramped barrel as mentioned above, chamber reamer. Some barrels come pre-cut but they have shorter hoods. you'll want a proper fit.
Tips, find a cone/bull barrel if you can. There a bit of a pain to fit but give excellant lockup.
If you don't go that way try the Briley bushing...love those things. you can change bushing rings out when they get worn and fitting is a snap. simply choose the correct size and put it in. fitting the of the bushing is still the same. Briley Manufacturing, shotgun choke manufacturer and gunsmithing - Pistol Parts and Accessories
they make great barrels too.
don't push the upper locking lugs entirely into the slide...don't recall exactly but I thing it's about .050 lockup. There is a tool for lining up the barrel that helps.
get a lower lug cutter for the feet...will help get that tight lockup.
crap, don't have time to get into all the details need to get back to work...building maps. wish I still built guns