Fueling & Injection - fuel pump check valve faulty ?

Hey guys, I have a 94 LT1 that recently started having a problem with long starts. I've changed the fuel pressure regulator & fuel pump relay. Same symptoms. A shop mechanic told me its likely the fuel pump check valve. The car does start after I wait about 10 seconds, with key on, after a long sitting, but starts right up if driven with 10 minutes. Is it safe to install an external check valve, perhaps ahead of the fuel filter, and would it interfere and be an additional restriction ?

Get a gauge on the fuel rail & see if it is actually losing pressure.

It is pretty rare that the check valve goes bad. It is usually an intank line or regulator that causes this problem.
A check valve will not hurt, but may not help your situation.