ok, it's conclusive: i have 2 bad MSD boxes.

pat explained that the issue was in the rev limiting chips, that they were wildly inaccurate, causing exactly the problems i'm seeing.

we already had the points distributor back in, and so took it for a ride, and it behaved better, (was hooked to the MSD box), but still wasn't great, cutting out around 6k, where it used to cut out pretty badly around ~5600. finally had the idea to up the rev limiter, and so set it at 8500 or so. took it out and it ran great, well past 6500 and still going strong.

put the mopar electronic distributor and the blaster coil back in, and it ran the same, fantastic.

so it's the box and it's rev limiter circuits.

now i just have to see if i can get a refund for these boxes...