Not sure if this is the right place for this but here goes anyway!

I am looking for either a flip up front sight gas block combo, or a picatinny gas block front sight combo that will be at the proper height when using a rear sight mounted directly on a flat-top upper.

Anybody understand what I am asking for here?

I essentially want to remove the standard front sight tower and replace it with a gas-block and sight combo but do not want to run out of adjustment.

Any suggestions?

Check PKFirearms

PKFirearms

Any front sight designed for the AR will be the proper height to function with a standard or flip up rear sight.

Not so.........
There are gas blocks of different heights so it would be possible to have a shortergas block and sight combination that would be much lower than waht would work with a flip up sight mounted on the reciever yes?

Correct.  You have;
Lower profile gas block mounted FSBs and Quadrail mounted FSBs.  There are gas blocks at the same height as quadrails.  So you first need to know what style you have, then buy the proper FSB.