So I just installed some old cranks.......only to find that the right pedal threads are striped. I know it's a long shot, but is there any way to salvage these cranks? Are they destined for the trash chute?

It can be done.

If it isn't stripped too badly, you can usually just run a tap through from the inside of the crankarm and clean up the threads enough to make it work.

If it's completely stripped, you can drill an oversized hole in the crankarm, tap a thread and install a threaded insert bushing. That requires some uncommon tools so it might be a cost prohibitive thing to do just once and it requires quite a bit of work so it might not be worthwhile to pay somebody else to do it for you.

Is a tap a standard tool that I could get at an LBS? How much do you think it would cost?

Wholesale cost for a left and right pair is about $20.00. I have no idea what an LBS might charge on a resale, probably about double. You'll also need something to hold the taps nice and straight while you are using them.

Thanks for your help. What kind of tools would be necessary for this job? Would this bushing be available at a hardware store, or is it bike-specific?

Threading on pedals is unique to bikes, so it will be an LBS type thing. Standard
repair item in the past, getting to be less common now. Unless you are really attached
to the crank, it would be better to hunt around for a replacement. For low end gruppos of yesteryear replacements can be in the $20-40 range (less for L than R).
Some older shops have a number of takeoffs laying about for low prices. The actual process of repair is straight forward, drill out threads, tap for repair thread and screw in repair thread with heavy duty loctite. The inside of repair thread is already tapped for pedal thread. One brand is helicoil. Since pedal threads are essentially not used anywhere else the equipment is of no use outside a bike shop. Steve