im working on a 98 chevy truck with350. it come in with a miss on 5 and 7. changed cap rotor wires and plugs and injectors. same thing. 135 to 140 pounds compression. put air in 5 and it will not come out anywhere.i pulled the dist out the gear was worn. put a used dist in and 5 started to work but could not set the dist right. got one used dist and tried it same as before no 5/7 and could not get it set. took it gm and they could not get it set. they also reprogrammed the computer no good. any on got any more ideas? thanks for the help.

Any shop should have no problem timing that, it is computer timed. How many miles does it have.

110,000 miles

at 110k the cam should not be worn, but it is a possibility. Timing MUST be set with a scanner by the cam retard being between-2 and +2 and you must bring the revs up and let settle back to idle after every movement of the dist to get an accurate reading.
I fought this problem for a LOOONG time before figuring out the dist gear on my 98 was well worn. My cam is also worn, but not horrible, so the new dist gear was good enough for now. But mine also had 200k+ before showing wear.

i wonder if the intake gasket leak would cause it miss? those vortec's are notorius for the intake leaks although yours sounds more like a firing issue

the replacement of the intake gasket is many times the culprit as it is not timed correctly when the dist is put back in.

Have you checked the pick-up coil. I had a problem with mine missing out also. We replaced all the plugs, wires, cap, and rotor. Still missed out. Took it to a shop that had all the computer equipment to check the truck out. He replaced the pick-up coil and it fixed it.

pulled the intake and found number 6 push rod bent. pulled head and the exhaust valve was bent. have know idea why the scanner did not show number 6 miss. must have been sucking ehaust into 5 and 7 causing the miss on them. thanks for the help.