Well, it's several weeks later, and I just learned something just HILARIOUS...
Apparently my 904 was NOT fried and it still has 3rd-gear. I just never had a kickdown cable installed after I did my intake/headers and it shifted at the appropriate points (while slowly destroying itself in the process)... after having the Lokar kickdown cable installed, apparently the mechanic shop did not adjust it properly (probably just bolted-it-in) and it was simply shifting at the wrong points way, way up in the engine RPM range/higher street speeds.
I guess my old 904 still has at least some life left in it, and even they admitted that after a year of my (very occasional) abuse, the trany fluid that was in there was like new with absolutely no debris from "failed" parts when they had to drop my pan and install a newer pan gasket to replace my leaking one.
It took a few trips around the block since last Thursday and some research on these forums to come to this conclusion.
My $800-worth of trany parts will just sit in boxes until I actually need the transmission rebuilt, I guess. So much for the 2-hour wasted trip and lost $100 deposit on the V8 trany, also.
The moral of this story: Do your own independent research, the kickdown cable (or stock linkage if not using a custom cable like mine) has a VERY dramatic effect on shift points, and research before you spend $$$ instead of taking a modern-day mechanic shop that works on newer models 99% of the time at their word. Besides this, I will give my mechanic shop a pass, as I know they may have never worked on an old Mopar before (I know they have worked on 60-70's Ford's and Chevy's in addition to multiple Italian exotics, though). They generally did a great job on installing a new steering box, power steering pump, brake job, and custom gauges. I sent them all of the articles on how to adjust the kickdown cable properly on the slanty, so hopefully they will get it right now. At least they are not trying to charge me for the correction like some shady places do.