Tires for a 1976 Ford F150 Ranger?

Truck currently has P255/70R15. I'm looking for a shorter/lower tire that is still more than capable of taking the weight of the truck. Any suggestions?

70 is the height of the sidewalls, so hunt for something lower than that, 60, 50.

The problem with that is if you go with a shorter sidewall aspect ratio tire you would lose the needed load carrying capability necessary for your Ranger. The use of a 265/70-15 tire would be about as low profile as you could safely go. The suspension on your Ford simply was not designed or engineered to handle a low profile tire.

I'm guesstimating that your F-150 originally packed LR78-15 radials. From TireGuides.com, LT235/75R15, LT245/70R15, LT255/65R15, and LT265/60R15 are potentially usable, depending on the width of the stock rims. Low-profile tires satisfactory for a half-ton will mean 16" wheels of half as much additional backspacing as additional rim width to maintain scrub characteristics with respect to steering; e.g. LT285/55R16 tires would require 16"x9" rims with 6.0" backspacing.