On the original question, as it's a '68 383 and you want a little more from a dead stop nail-it, the FIRST upgrade from your allegedly stock converter would be for the "1968 Road Runner 383 4bbl" application. Easy as that. Forget about the stall speed specs and such, go with an OEM application that works and won't be too loose for normal driving, but you might need a little more steady throttle foot to no lose mpg under 55mph.
As for the leak, the ONLY places where a front of the trans leak can happen is where the front pump gaskets are (seal it to the trans case), the front pump seal (which seals the snout on the converter to the front pump housing (a lip seal), and/or the converter snout itself (as the seal might have worn a groove into it, from normal wear and tear).
IF they put that "reseal and gasket" job to one of their less-experienced employees, which might indicate just how easy that job might be with the right tools and equipment, then they are probably suspecting something wasn't done completely "right". Hence the free labor or "Shop Warranty".
In one respect, you might have budgeted for some items, but spending money unwisely is just the same as "over-budget" or "down the tubes", to me. I know it can be difficult to chose a repair shop that will even work on any older rwd car, BUT there should be some car clubs in your region which you might join and network with in order to find a good (don't like that word "reputable" as that can mean different things for different vehicles) shop. EVEN if the shops might be an hour away from you, it can hopefully be worth the effort to get to them.
Not to say that you didn't do some good research to get to these shops, BUT perhaps the right questions weren't asked? Perhaps expectations are a bit too high? Not sure. In my own perspective, getting a job done to the "OEM Production Standard", in parts and labor, should be the minimum standard you request of them. If they don't feel they can achieve that, kindly thank them, say "Let me do some thinking . . .", and leave. No drama.
The OTHER thing is to see for yourself where any issues might be BEFORE taking it into the shop! Don't completely rely upon what they tell you might be wrong. Do YOUR own research first, so you have a better idea of what's involved and can ask questions BEFORE anything is done.
Seems like I recall you saying the prior owner stated the engine had been rebuilt with "a little better cam", of undetermined specs? Will it still idle at 550-600rpm in "N" and go into "D" smoothly or does it need more rpm to idle smoothly in gear? Just curious as the term "a little better" can be "just a bit" or "more". Or it could be an aftermarket cam that might have larger numbers than the stock cam, but really isn't much "better" at all. Which is why I mention the idle characteristics as a guide.
Do NOT take it as a Badge of Valor as to how much you might spend on this car. Spending wisely and effectively, prioritizing as you go, can work better with better results. Once you find a mechanic that'll help you learn about the car, how and why it works as it does, that will take pride in helping you as he enjoys helping you along, that's the one you need to stay with. ANYBODY can take your money and such, but the one who might take less and give you more (education in the process) to make the car work better, is probably one to patronize. I've seen "good mentors" over the years and they are usually the ones you might overlook, but can have great knowledge they can pass on. You just have to find them, but they are there.
Enjoy!
CBODY67