Don't give up Tim. Don't think of it as being over your head. You're just learning new skills.

Rusty floors are a common problem around here. If you can find a supplier of replacement floor pans you should buy them. It makes the job much easier and faster. The 49 Plymouth isn't as popular as old Fords or Chevys so you might have to make your own.

I'm not sure what gauge steel I use but when I need to make floorpans I go to the dump and get a door from an old refrigerator or skin an old washing machine. It's heavier than the sheets of body steel they sell at the parts store and it's free.

Use a sheet of cardboard to make a pattern for the floor. For difficult areas I use masking paper and cut it to fit and mark where it needs to be bent. You can tape these to the cardboard to make it one piece or cut them out seperatley and weld in one piece at a time.

Butt welding is best in theory. I wouldn't recommend it to a beginner. It's very time consuming making everything an exact fit all the way around and nearly impossible.
I put the pan in place and trace around it with a marker then pull it out and cut about 1/2 inside the line for a minimal overlap. Then grind off the paint or rust from about 1 inch around the edge of the hole. Some people just tack it in with a mig and use body sealer between the welds. I like to use the torch and gas weld all the way around the edge. When it cools I go under the car and paint it with rustoleum making sure I push it up into the overlap with the brush. When it dries you can put body sealer in there just to be safe. I usually just seal it with rubberized undercoating.