the idea with the 275's would be so you only "need" to purchase 2 more tires. that and they are fairly common sizes paired together... as with the staggarded rims, the tire "bulge" is pretty close front to back.
one of my past projects ran 285/30/18's on a 18x9.5(that was a 4 hour long PITA when i mounted them), with that size there is zero sidwall bulge....too much bulge seems to translate into sidewall flex, even with lower profile tires......good for potholes...bad for consistant feedback. i usually try to match the tread width to the overall rim width, then adjust accordingly to the sizes available.
sounds like you got it under control...hope we helped