judging by the other questions he has asked about his ranchero, (smoking, knocking. oil leaks,no fan belt) I seriosly doubt the original rims are still on it. 14 will fit but if you want performance tires, most choices come at 15" and up.post a couple pictures of your new buy, we could help you window shop!
Late '70's Rancheros came standard w/14''rims. 15''were optional. If your lucky, your Ranch. WILL HAVE THE tire placard on the passenger side door jamb still. It will give you tire sizes & recomened optional sizes along with tire pressures & load limits.
even cheap gauges are better than idiot lights. easy to install. I use oil pressure, vaccuum, and coolant temp in my 67 fairlane. but remember you get what you pay for. my 'el cheapo' temp gauge sometimes dosnt move and I have to tap it to free the needle. autolites are sweet, and kinda pricey, but cheeaper than a new engine.