275 is fine no a 10" rim depending on the brand. Most BFGs will fit great, something like a Nitto or a Sumi will be stretched.
Here is a BFG 275 on a 10" wide rim
As you can see, the fit is perfect but this particular BFG 275 is more like a 295 or a 305 in Sumi/Nitto size. You can check actual widths and other specs on tirerack.
As far as a 275/35/17, that is WAY too short of a tire, the middle number, 35 in this case, is not a static number it is a percentage. 35% of 275mm. The correct OEM height of 25.7-26.1 for a 94-04 Mustang is achieved with a 275/40/17.