I'd say a 26x10x15 is a bit smaller than a 275. But, that is just looking at the hard numbers on a ruler. Actual application with tire design, compounds and rims can alter this some. Double check your section widths to make sure. If you have 235s on there now, verify its section width agains the 10" M/T claim to see how much larger you actually are going to be at, because I'm thinking those M/Ts are bigger than a 245.

Also, I'd say that the 28" rear tire is going to be a bit tall. The original TA racing slicks were a ten inch tread on an 8 inch wheel. Modern comparison would be like a 285/50 on an 8" rim, so an M/T 26x12x15 may be closer in appearance to a vintage T/A look than the 28" tire.