18 Inch Wheels 78 Trans Am

[68Formula]

I’m working with same issue right now.
1978 TA restored try to hit 10x18 with 295/40R18 with ridetech streetgrip leaf springs.

Current wheels are Year One 9x17 with 275/40R17 BS 5.125 and plenty of room in wheel well, factory height now.
Same size in front, sits nicely to ”corners” no rubbing.

Regarding rears, first problem noticed, looks axle is almost 1/2” more on passenger side so needs centering.

measured/compared with current wheel tire combo, 10x18 offset 0 BS 5.5 should fit to my car with 295/40 and 2” lowering, may require light hammering inner wheel well where’s seatbelt mounts.

fronts will go to 8.5 offset 0 and 245/40R18.

If you can fit a 275 on the front, why downsize to a 245?