Sorry to be late getting to this thread.
The ROUSH filters have a slightly thicker casing than an FL1A , but not as thick as a the HP. --It also has a
Synthetic filtering element
Unless I am missing something -- I dont see your pressures going to the point of needing anything beyond the Roush filter. -
The Roush burst pressures are probably inline with the WIX 51515 if not better , and that is the big difference --
The Burst Pressure Rating of a FL1A is about 290, the FL1A-HP is rated at 500psi.
Other than that is all about GPM and Microns. 7to9GPM and 28 +or - microns
Another set of Geek Food would be this;
WIX Crossover for FL1HP
http://www.wixfilters.com/filterlook...asp?Part=51622
WIX Crossover for FLIA
http://www.wixfilters.com/filterlook...asp?Part=51515
Steve
Another Substitute is a wix PN.