<VV> Engine problems

[Kinzelman, Andy]

Guys,
We've been working on getting a 65 Monza up & running; rebuilt the brakes, new tires, O-rings, cold valve adjustment, professionally rebuilt carbs, points, & plugs.  On a test drive it ran great, until the fanbelt came off (no warning light).  By the time we got back to the garage (within no more than one mile) the engine was running fast & clattering.  We put the belt back on, but could not get the car to restart.
I was told that the next morning it started right up & ran as normal - for about 15 seconds.  Then it started to die; revs dropped, & shut down  - in about 10 seconds.  No amount of cranking would get it to restart.
We pulled the plugs & did a compression check - all holes appeared fine (135 - 145).  Cleaned & reset the points, static timed it, new plugs (properly gapped), checked for spark, ensured the rotor was turning, ensured the advance weights weren't sticking, charged the battery, looked the carbs over, checked the condenser, & made sure the plug wires were in order.  After much fiddling had the car start again.  It acted just as it had the first time - like it was being choked off.  I pulled the crossover & muffler (thinking something could be plugging it up), but this made no change.  I also checked that the HB hadn't slipped - the timing mark lined up OK when #1 was at TDC of the compression stroke.

When it cranks it fires occasionally (like it's flooding out, or mistimed), but it just doesn't want to start.

My next thought is to look at the valve train (maybe the valves are not operating correctly).  Otherwise we're stumped.
Anyone have any ideas?  I really don't think we damaged the engine; it wasn't that hot.  However, it's extremely frustrating that we can't get it running again.  I don't want to swap in another engine without determining that this engine is damaged.
Thanks,
Andy K.