27" tires usually means 27 x 1 1/4 or 27 x 1 1/8 there's a few tires out there made in this. I needed something as close to punctureproof as possible, so I got continental gatorskins. pricey but nice.
I'd guess from the age of the bike its got a one-piece crank. you can get a product called kneesavers which will adapt the 1/2" pedal threading to newer 9/16 pedal threading to fit clipless pedals. that's $45 usually.
your other option would be to get a new bottom-bracket and crankset along with pedals, but that usualy needs a $25 threading adapter. it would be more expensive in the long run to do this but you could choose a chainring to help get the heavier bike up the hills.
one thing I'd generally recommend is to overhaul (clean up and re-grease) the hubs I did this to my '77 schwinn and it helped tons.
good luck