I installed a new blower motor for a '94 into my '80 this summer as well, and I took advantage of an opportunity to take some measurements with a calibrated anemometer before and after the upgrade. I reported my results in this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=594506
Basically it was a bit of a disappointment in the sense that the increase in flow was barely perceptible, but it was measurable. Given the very low cost of a new blower motor I decided it was definitely worth the money spent. For $20 dollars or so I realized nearly a nearly 15% improvement in air flow.
Perhaps my blower had been replaced years ago, and if it had been the original stock blower from 35 years ago I'd have seen a much greater improvement. I'd say if a person has any doubts about their blower, just plunk down the money and see if things get better. But unless you have a way to measure the difference, it is very possible you won't be able to tell.