Op I’m reading back over this. When you say the tube is longer I’m assuming you’re meaning the receiver extension (buffer tube) that came with the UU heavy kit?
If so that receiver extension is larger than an ar15 receiver extension. It is the same internal depth as an A5. The added length lets you use a standard H buffer and spring (like the one in the UU heavy kit) instead of a short ar10 buffer.
The spacer people are talking about would be used in an a2 rifle receiver extension with the included H or H2 buffer and springco spring. With the rifle receiver extension you can use an ar10 rifle buffer and spring and you wouldn’t need to use the spacer.
I hope this helps. Sorry if I didn’t fully understand your question.
Personally I’d never use the short ar10 buffer on an ar15 receiver extension. I’d also never use the spacer when I could use a rifle buffer and spring.
Probably the easiest is to do is sell your PRS and just drop this guy on
Or get out the dremel
https://www.larue.com/products/b5-precision-stock/Not my picture.