Does your car have it's factory wide rear end ?
Do not know anyone who makes a 295/50/15 tire that has a Side Wall Width of only 10", who is the tire maker & model # ??
A 15" x 8" Rim with a 4.5" to 4.75" maybe 5" BackSpace has been known to fit other guys cars,
but it all depends on how much room there is in your car's Fender Wells.
Backspace of the mounting rim and the tires Side Wall Width & Diameter play the biggest part
in whether a 8" or 10" rim and a big tire will fit within your cars Fender Wells.
The stock factory Rear Ends in 68 - 72 Chevelles are wider than the 64 - 67s.
A. First you should clarify which Rear End width you have in your car .
Here is the source for rear end widths http://chevellestuff.com/qd/generic_rear_axle_info.htm
* 1964-67 Chevelle rear ends were manufactured by the Chevrolet Gear and Axle Division.
The housing measures 53.25 inches from backing plate to backing plate
and with the brake drums in place, the total width is 60.00 inches.
Axle tube flange-to-flange is 54.25 inches.
* 1968-72 Chevelle rear end housings measure 58.5 inches from flange to flange,
and with the brake drums in place, the total width is 62.5 inches.
(Note: Other sources put the total width at 61.125" to 61.25".)
Axle tube flange-to-flange is 55.25 inches.
* A second measurement to check is centerline distance between spring perch seats.
1964-1967 are 35 1/16 inches between seats
where 1968-1972 are 34 31/32 inches.
B. How much room do you have in your Rear Fender Wells ?
eg. from outside to inside, Fender Lip to Frame/Wheel Well
from front to back of outside part Fender Well
By measuring the room you have in your fender wells from the inside of
the outer finder lip to the frame/ inner finder well for width
and the max room from front to back within the fender well as well .
eg. Basically there are various measurements to be made within the rear fender wheel wells;
1. RIM SPACE inside curved lip of Rim to bottom part of Shock Nut for DR & Pas Sides
(DR Side = ? ” PASS Side = ? ”)
2. OUT SPACE from inside Lip of outside Fender to Side Wall of Tire for DR & Pas Sides
(DR Side = ? ” PASS Side = ? ”)
3. IN SPACE from Side Wall of Tire to Frame/inside face of Fender well for DR & Pas Sides
(DR Side= ? ” PASS Side= ? ”)
4. WheelWellSpace from inside of bodyfender to frame/fender = ?? "
and from grd to top of the inside of fender well = ?? ” + height / diam
5. SPACE from Face of Rear Brake Hub to Shock Nut = ? “ DR & PASS sides
6. SPACE from inside of Fender to Face of Rear Brake Hub = ? “
7. Rear End Width from face of mounting surface on Drum/Disc Brake DR side to PASS side = ?? "
8. SPACE from Front of Fender Well to front of current tire when mounted to rear end .
C. What are your current rims & tires measurement details ?
eg. rim width & backspace / tire Sidewall Width / Diameter (height of tire)
The Backspaceing of a 8" or 10" rim will depend on the measurements of
your Fender Wells width & height and the width of the rear end you currently have in your 68.
And each side can be different !
Other items can also obstruct your rim & mounted tire,
eg. outside front part of your Fender Well if the tire is to hi / rear lower shock bolt / exhaust pipes etc .