im running ridlers at the sizes your looking at on a 70 fleetside with the wider of the 12 bolt rear ends. 57" backing plate to backing plate
running 275/40/20... i think there is enough room for 295/40 but again this is the wider rear end.
axle width is 55.5 wide with 30.5 long axles on the narrow rear end. earlier rear end
axle width is 57 wide with 31 5/16 long axles on the wider rear end. later rear end
this site is helpful in seeing sidewall measurements and what fits on rims
https://www.rimsntires.com/specspro.jsp