No idea where to start

Consider yourself lucky on a '67. My 74 was pretty bad in this respect.

Cut it out, weld in (or glue in) new.

There's a few schools of thought on this repair:

The purists say take the time and butt weld everything.

A little trick: You cut the new piece to fit 1/2 or so over your perimeter, then with a grinding wheel or sawzall, cut 3 inches or so on your line thru the old metal, then tack, cut 3 inches tack etc, until the whole old metal falls to the floor and new is in it's place with matched even level joints. Drill holes every few inches where it connects to the sub frame, and weld thru those holes, into the frame.

The get-r-done camp say lap the joint drill a hole in the new metal every 2 or 3 inches, spray edges with weld thru primer, lay it in, then weld thru that hole to the base metal, also putting rust inhibiting primer in between the lapped pieces, and fill the rest in with construction adhesive. (The home depot kind works great!) Drill holes every few inches where it connects to the sub frame, and weld thru those holes, into the frame. It is also a nice process for thin metal you don't want to blow thru with a weld.

Or just cut it, and glue it in.

All acceptable ways of doing things, the last option, more on the temporary side I'd say, or if you don't have access to a welder. The main thing here is to make sure it doesn't rust where you fixed it. Seal it up nice. Its also a good starting point for other areas of body work. You can cut your teeth here and it doesn't show.

New floor pans can be had for $50 Including shipping on e-bay, if you want to go that route. Sounds like a few pieces of scrap 18 or 20 ga will work fine for you.

Its your car, have fun!

Hope this helps.