1971 C10 Tire Size for 15x7 wheel?

[embr03]

I'm looking to improve the stance on my truck. Lowered 4in springs in the rear and 2.5in spindles in the front. Currently have 15x7 wheels and 265/70R15 tires on all four. Was thinking maybe 255/60R15 in the front and 275/60R15 in the rear? Ran into issue with Discount Tire shop that they couldn't install these because of the loading limit on the tires, but another tire shop said they could. Or should I go for 235/70R15 in the front and 255/70R12 in the rear?
Any advice and pictures of your setups would help a lot. Thank you!