I use a 6 oz carbine buffer with a rifle length recoil spring. What you're going to need to do is. Once you have the buffer and spring installed, is charge the bolt carrier. Observe how far back if at all it travels. You may need to add a buffer spring spacer behind the buffer, inside the recoil spring. Once i remember the measurements i'll edit the post. .
As for the upper. They don't come with feed ramps and not all bullet profiles will cycle, or feed. You should order this unit, just in case. It can be installed in the upper using a good flat file.
https://taccom3g.com/product/taccom-...ower-feedramp/
Here's taccoms site for more goodies.
https://taccom3g.com/