Re: Backfiring. Is the popping in the intake, or in the exhaust (aka afterfiring)?

Re: Ignition timing. Is the distributor providing appropriate centrifugal advance? As a check on timing, remove both vacuum lines from dist. and plug them. At idle ~ 10deg. BTDC. At 1500rpm ~ 20deg. BTDC. Above 3000rpm ~ 30deg. BTDC. Advance vacuum should add ~ 12 deg. for a total of ~ 42 deg. above 3500rpm.

Re: Cyl. head rebuild. Has the cam timing been checked and verified?

These questions are asked because errors in both cam and ignition timing can lead to chasing after injection "problems" that aren't. That said, if cam and ignition are eliminated, the next most likely source of the symptoms you describe is in the WUR. If, as the WUR increases in temperature, it closes too far, the control pressure will become too high and the metering plunger in the FD will be held too far in the lean direction.