Bad night of racing/ bent valve

[JB400]

So, my uncle just got done rebuilding a 2.6L 4 cyl. for his Conquest mini stock.  He installed an aftermarket aluminum non-jetted head complete with stainless valves, different valve springs, a Caravan cam, adjustable rockers, and flat top pistons.  Everything else was stock, minus the counterbalance shafts.  We had it internally balanced to eliminate those.  Everything was built to specs, according to our book, unless it was overruled by the manufacturer of the aftermarket parts.  We got the engine fired up, broke in, and had no problems.  No pings or knocks or any other strange noises.  We got the timing set and had the engine idling at a mellow 1,100 rpm.  Revved it up to 5,500-6,000 rpm multiple times with no problems.  Had real good oil pressure and water temps.  

Race day:  We checked off everything on the checklist and even drove the car around the block without a problem before loading it on the trailer and heading to the track. We got to the track (1/4 mile dirt track) and went through the checklist again while waiting for hot laps. After warming the car up for a couple minutes, my uncle goes out for hot laps.   Car sounded great and held its own.  Engine sounded healthy when he pulled in.  His heat race was first up after the 4 other classes run their hot laps.  His class pulls out, with my uncle on poll position.  5 cars in the race.  The field gets the green and goes in turn 1, 3 wide for the lead, with my uncle in the bottom lane.  He drifts up into the guy in the middle, mashes the gas, and gains the lead pulling out of turn 2.   Just before he turns to go into turn 3, the engine goes blah.  He pulls into the pits, with car barely running.  We pull the air cleaner off, and fire blows back through the carb.  Pull the valve cover and one of the rockers is loose on the intake side.  We tried adjusting it, but it didn't get any better. Called it a night.  Good thing too.  The track ended up going to pots.  Several cars ended up with bent rims, blown steering racks, and a lot of tore up cars.  Turns out the track owner put fresh clay on earlier that week and didn't work it in good enough.

We took the head off the car tonight to find that we bent an intake valve in the #4 cyl. It left a pretty good   on the face of the piston.  Does anyone have any ideas as to what we might have done that would have caused this to happen?  Everything else is fine.  All the other valves clear with no problem.  Did we have a valve spring go soft on us?  We're sort of   and   at the moment.

                                                                              Thanks for any insight

[fy469rtse]

I would post photos and pm challenger340,
I would like to see photos, ? Impact mark on piston , it could be many factors, did it spit that pushrod,
Who back cut valves , I could go on and on without seeing it ?