340 duster only good for a couple of burnouts.....

Well that kindof rules out lash then.When the valves fail to seal, the engine normally stalls when you bring the Rs down, and it takes several minutes or longer for the lifters to bleed down for a restart.
Maybe the tires are crap when cold,heehee.

Ok here comes a test.
With the engine Cool(not cold), run it up to 4000 in first and the run second to 70mph. Run it as hard as you can without wheelspin.Have a helper time the run with a stopwatch.. Repeat.Total the times together. Now go drive it around til it gets tired. Then repeat the test,with the same helper,again two times and total them up. Compare the two results. If the results are the same or very similar, then the engine is still putting out about the same.
But if there is a very significant difference, say 15% or 20%, then either the engine is down, or it is more heavily loaded.
So the first thing to do is come to a stop,slam the car into neutral,jump out and push the car on flat level hard ground. If it pushes easy, at least your brakes are not dragging, and neither are any tranny guts.The reason I mention this is because I have seen front calipers stick on after a bit of driving, but overnight they may release. This is usually pistons sticking after a bit of heat gets into them,but can also be a flexhose problem. I have also traced this to a compensating port issue, and boiling brake fluid.So don't think I'm coming out of left field with the push-test.
You can also do a coast-down test, the same way. On flat level ground, on a calm day,with the cool brakes this time, run it up to 70, and put the tranny into Neutral.Then time the coast down from 65 to 45. No brakes, repeat in reverse direction, add the numbers together.
Then drive it around again til it gets lazy and repeat the coast down test again two more times. Compare the results. The times should be the same or very similar. If they are significantly different, I would immediately stop the car, jack it up and check for brake drag; both fronts. No drag means the power loss is in the tranny or in the rear. So hustle the jack out back, and crank the rear end up. Put her in Neutral and spin the tires by hand. No drag here leaves the tranny or the TC.But a drag here could be in the brakes, the flexhose,the C&P, or the driveshaft.
I can't imagine a drag in a TF but I could imagine a bad TC.
I could also imagine a tranny failing to downshift into first when coming to a stop :(