With the proper offset (backspacing), a 275-60-15 (15" diameter, correct?)... is no problem at all.
A 295-50-15 (hard to find) can be a problem pending the rim, would be pinched at the sidewalls... but again the offset (backspacing) is critical.
ZERO offset should be the magical number (keeping the rim centered within the wheelwell).
Do you know how to measure the backspacing?... Measure the entire rim width outer-to-outer, then take a block of wood or straight-edge and lay it across the rear... then measure in towards the back of the wheel where it mates the brake face hub (drum face).
IF you truly have an 8.5" width rim (advertised), chances are the overall width will be about 9.5". Divide that by 2 and you'll get 4.75".. which would then center the rim (zero offset)... and keep the wheel centered within the wheelwell. A 275-60 should fit easily... probably NOT even needing the fenderwell lips to be flushed (but that's a good thing to do for peace-of-mind pending a soft suspension).
Realistically, you can draw this all out on paper and see the effects that happen with different setups. I believe there's some apps that can do it for you.
Note: I'm currently running 18x11 rims , 5.5" backspacing... and 295-30-18 tires front and rear... lips flushed... stiff autocross suspension.