SPB ROM??? What is that? The letters "SPB" are normally associated with the company SPB Software House (
www.spbsoftwarehouse.com) and they make software for phones...not ROMs e.g. SPB Mobile Shell.
Regardless, if you flash a custom ROM e.g. MightyMike, Energy, Juicy, etc. you'll be flashing a brand new OS, like 6.5 as you mentioned. Typcially you'd then either try different ROMs, or if you find one you like you'd probably flash the new versions of that chefs ROM as they are released / updated.