So was it doing OK on the heat soak starts, but recently started this action or is this a long term problem?

The first thing I'd do is get it warmed up as normal and then give it the 20 to 30 minute resting time. Before trying to start it pull the air cleaner, get up where you can see down the carb venturis and give the throttle a couple of full pumps to prove that you've got gas in the bowls. I'm thinking that you might be leaking down the fuel, or boiling it away with the heat soak. If you don't get a fuel shot consider a thick phenolic gasket to isolate the carb base from the intake. It could be that you're leaking all of the bowl fuel into the intake and it's flooded? Maybe try hard WOT during crank to see if it comes to life quicker with more air?

If the carb seems to have gas, given recent experiences with spark plug QA/QC I'd invest in a new set of quality plugs to rule out the possibility of them breaking down with heat. I like NGK's, and I now avoid AC's like a plague after watching the video of the China sweat shop making them.

If the plugs don't change anything then go through the warm up & heat soak again, pull a plug and with the plug well grounded have someone crank the engine and observe the spark at the plug for a strong blue/white spark and a distinct "pop" from the MSD. If the spark is weak I'd probably spring for a new hot coil, like an MSD Blaster.

It's one of those trial & error things, trying not to simply replace a bunch of parts helter skelter. Let is know what you find, or what you've done to date that might change the approach.