Pontiac Repair: Engine stumbling, blown head gasket, code p0300

Question
1999 Montana 180,000 blown head gasket I change the heads were checked they are fine 2 push rods were broken I change them the van starts great just that it stumbles and the check engine light comes on with code P0300 plugs were replace with AC plugs  can you please help me

Answer
Hi
I would have data observed and pinpoint the EXACT  
cylinder(s) that are misfiring.
From there you could attempt to determine if it is a Coil, plug wire, ignition module, fuel injector, any injector related wiring or something improperly repaired mechanically.
I REALLY would like to see broken push rods. Bent maybe but broken... hard to believe.
Plus were they bent/broken before or after work COMPLETED.
If after, they MAY have been installed in the wrong location and when attempted to start, they bent.
If this happened, the valves are LIKELY bent, and that could EASLIY cause your p0300. The intake and exhaust push rods are different lengths.
I hope this helps
Pawl