Seems to me like one way to fix the fuel pressure
fade problem and rail imbalance would be something
like this:

- Tap off fore ends of rails, add a front-end "Y"
fitting, run Y to a Ford blowoff regulator.

- Run one hard return line back to the pump and
"Y" it with the main regulator return (is this
accessible? Dunno).

- Use Ford SVO injectors at their Ford rating, to
match whatever flow you previously had at the
GM pressure.

Since the GM pump runs a higher pressure, the Ford
blowoff will have plenty of headroom and will be
able to maintain a flat fuel pressure at the head
end of the vehicle. The union at the front would
also improve rail-rail balance.

This would be most pertinent to a car where you're
already fixing to change injectors out and don't mind
sweating some fittings onto stock rails or tapping
some aftermarket ones. But it would require only
a few parts and routing one line back to the tank
for return.