I like the rubber gaskets and no sealant. With a cast pan wither should seal well the trick is not to over tighten it, if you don't have a feel for it use a torque wrench and use the listed torque readings. you should be able to find them in a Chiltons or Motors, they are going to be about the same as a rocker cover.
the big thing with the TH350 is to make sure it's not leaking from one of the other dozen places it is likely to leak from, besides the pan gasket. Like the dipstick tube, kickdown cable, speedometer drive, shifter shaft, accumulator cover, front seal, front pump O ring, front pump gasket, front pump bolts, extension housing O ring, etc.
The fill tube and kick down are really common. I cannot even count the times that I have fought a filler until I was ready to cash it in in a non stock setup. I actually put a locking dip stick on one for a fella once and ran it through the floor board, shortened of course, told him all the racers were doing it. he was happy as a duck and his leak was gone. Funny he got lots of good comments on my shade tree approach to making his filler tube not leak.
I always use a rubber gasket with a cast pan and a cork gasket with a stamped. If using a stamped pan, take a straight edge and check the pan rail to make sure its flat around to holes. I've never seen one that wasn't a little tweeked. If it is flatten it out and torque it lightly so you don't crush the gasket. I've have used gaskets that have small steel sleeves in the bolt holes so you can't overtightened it.
I have always used those springloaded aftermarket pieces which resemble a long washer that lay on top of the valve cover flanges. No solution for pan or trans pan leaks other than very careful tightening
I don't use gaskets any more, I put a bead of Permatex "The Right Stuff" on the pan, cinch it down, and no more leaks.. When it is time to change the filter it will allow the pan to come off and you can scrape the old stuff off pretty easily with a razor blade. Don
I ordered a Duraprene gasket and used no sealant. I checked my pan for bows and leveled them with a ball peen. The pan is chrome so I scuffed it with a 3M wheel on a diegrinder. The bolts were finger tightened and then torqued to 120 inch pounds. No leaks. This was my fourth attempt. Looks like 4 times is a charm.