The tank check valve is a C.C. required deal for all top draw tanks. I would still go back and check fuel pressure at WOT and you must install the F.P. guage as close to the carbs as possible for an accurate reading. When you say the 460 is stock am I to assume it has the D3VE-A2A heads on it? If so then you might want to bump the total timing up to about 35-36 @ 3000-3200 rpms. The engine should have no problem handling that timing as the C/R is only about 8-8.5 to 1 depending on block deck height. I'd also set the initial timing to 15-18 deg without exceding the 36 total. One more thing, get rid of those 3.5 power valves as they may be closing at your maximum rpm leaning out your engine. 600 CFM carbs are IMO too small for a 460. Try a vacume guage under the carbs to check manifold vacume, if the vacume starts to rise as you approch your maximum rpm the power valves will close cutting off the fuel from the PVCRs and you changed to a 3.5 p/v which will close sooner than the 7.5s you removed.
Unless that check valve is inside the tank then there isn't one there. As for the power valves I have rarely ever even dealt with them because I have always removed them on my blower motors. It seems that everyone has a different opinion of what to run for a PV on this application but I am about to the point of eliminating them and jetting up to compensate for it like I have done in the huffer motors in my 32 Dominator. Aside from the blower surge they have always idled just fine.
As for stepping up in carb size,that isn't an option so these have to be made to work as good as possible.
Also the fuel pressure gauge is located right underneath the carbs and is secured to the tunnel ram directly.