Yeah 300 is ALOT of p to v. He probably had the lifter bottomed out.

You need to find a machinist who uses another method to determine the cam phasing. Watching the lifters move is NOT the way to determine if the cam is in right.

What was the engine temp, thermostat or not and if so who's t-stat housing are you running?

What diameter are the valves?? may be expanding and kissing the piston..did they fly cut the pistons? when you say .300 is that intake or exhaust...i would also get rid of the edelbrock carb and find a 800 cfm carb..if its lein all sorts of chit happens to the valve trane from the heat.

Im with griff. Timing marks may be lined up, but did he installthe crank timing gear correctly? If its one of the deals with multiple degree positions that need to match cam gear. Seems like the timing is off causing it to run hot and cause some major valve dragging combine that with the valves opening and closing out of time with the pistons.... Disaster.

They have told me that they double checked the cam timing today and it was okay. I'm not so sure that they have degreed the cam. I believe that they double checked that they have put the timing gear together in the right holes and I've not done this myself before so I don't know what to look for. Coolant temps were 145 to 160 while underway before it smacked the valves. The machine shop has told me that they believe old fuel gummed the valves up and caused them to stick. Fuel was purchased last October and was stabilized supreme (91 oct rating). I ordered the Manley severe duty marine valves to install on the exhaust side as well as the corresponding Crane valve springs. I have also purchased a brand new Holley 800 double pumper marine carb. I'm frightened to death that I'm throwing good money after bad here.

GO TO A DIFFERENT MACHINIST!!!!!!!!!!

You are dealing with a guy that's "in over his head".

This isn't a stock 350 going into a Monte Carlo.

Bad gas making the valves stick?????? Please!

As far as the bad gas goes, I don't think it's old enough yet, but you could drain it to be sure.
A short story on stuck valves, a friend bought a '98 GT Mustang, it had been sitting for a few years, drained the tank, tune up etc. Ran pretty good, the next day he went to the shop to work on it some more and when he turned it over it had 0 compression. Most of the valves were stuck in an open position. Pulled the heads and it was like syrup (ethanol) in the guides. Re did the heads and it runs fine.
Not to hi jack the thread just wanted to share.

You have,,,, pushrod, rocker arm, timing, clearance issues. Meaning you need to start from the lifters up, make sure the pushrods are the right length, rocker arm ratios are what you need, and 300 thousands clearance,,,not sure about that! What timing set is in it? Is it adjustable?
Just a few thoughts.

Make him dyno it before he gives it back again.

But I agree,,,, find another assembler

The machine shop should be able to show you a build sheet on YOUR ENGINE with how YOUR cam is degreed in. I mean, they had it apart a few times, right, AFTER piston:valve issues?
Did they REALLY say "look at that lifter start to move, that means it's degreed properly"?!? WOW!!! I mean, Shizz son... maybe if Smokey Yunick said it, but he's dead so everybody ELSE shows the build sheet!

The Piston to Valve clearance must be done with a SOLID/testing lifter or similar, or the hydraulic pumps down, does NOT push the valve to full "pumped up lifter" lift, and shows falsely high piston to valve clearance.
I don't care WHAT they SAY the static P:V is! When your engine runs, MULTIPLE valves are RIGHT ON THE EDGE and some are going into INTERFERENCE condition. Your engine isn't "Magically" doing this, it is a setup issue of SOME kind.

You should not have SO much gum on the valves/valveguides to stick a valve against valvesprings with 250-350 lbs range seat pressure. If the valveguides were too tight and valves get hot that CAN happen but geez. Again, they should know the valveguide clearance figures. Not where the money is, I mean what kind of oil was being used that so much built up? It wasn't just ONE valve, you've had multiples.
If they were that shizzy, why didn't they disassemble the heads, clean them, clearance the valveguides/valves?

This is a MACHINE SHOP'S JOB, to BUILD you an engine COMPETENTLY that does not have basic MECHANICAL INTERFERENCE ISSUES, whether it's "dirty sticking valves" or "cam degreed wrong" or "heads/block were shaved/decked too much so less P:V clearance than should be when lifters pump up" or WHATEVER.

Again, you have MECHANICAL PARTS running into each other during rotation. This is the MACHINE SHOP'S JOB, it is NOT a "tune issue" or a "gasoline issue" or an "operator issue".
We aren't saying the shop are bad people but if you are PAYING them every time for this, "Their Freshness Date Has Expired".

I did just this with brand new inconnel valves. It is easy to install the crank timing gear in the wrong keyway and that is what I did. You description is identical to what happen when I had the crank gear in the wrong keyway. I'm with griff and meangreen. Does your crankgear have 3 keyways and if so which symbol key is the keyway on? Should be a triangle, o and 0.