Just curious why you need that extra .75 on the total external length? Your stock will fit the standard milspec tube at 7.25 just fine. Its the internal depth that's important for function. A VLTOR milspec tube should still be those same dimensions. I have one at home, I'll try and confirm.
If you are talking about the VLTOR A5 length receiver extension, it is a little longer internally but fits milspec sized stocks. Same length as the armalite AR10 carbine tube, and designed to be used with a special A5 buffer and spring on an ar15 rifle. I'd personally stick to the standard milspec extension with the standard 7" internal depth, and add the thick shoulder pad to the stock if you need the extra length. No point in rigging stuff with quarters or spacers if you don't need to.