You asked about the biggest tire but then just mentioned rims. With the relocation kit that you have 10.5" section width is max and it will require the correct backspacing on the rim.
OP your not listening;
If a 10.5 section is all that fits in there, then;10.5 x 25.4mm =266.7mm.The closest size to that is a 265; and at a .7 checking rim, that woulda been on a 7.5 inch rim. If you put a 265 on an 8inch rim it will grow perhaps 6 to 10 mm making it a 271 to 275mm or 10.7 to 10.8 inch section. But if you mount it on a 7 inch rim the profile might diminish those same 6 to 10 mm, making it a 259 to 255 section,or 10.2 to 10.0.
Now to make this successful with an 8incher, it will have to be very carefully centered.....And here's the trick; you can run the tire sidewall pretty close to the spring,say 1/8. But you have to stay away from the spring side of the tub if you are into cornering. About a half inch if the tire is well supported. But you for sure need to stay away from the fender lip; 3/4 to 1 inch;depending on the body sway, and rearend side-shift,which increases with higher arched springs. But the fender lip and inner tub wall is not always the trouble spot. As the tire gets taller it starts to rub on other things in there. And for straight-lining, a tall tire is what you need in a Dart tub.
Now heres another thing; you need to consider your driving style. The profile on the 8 inchers will not give much,if any, wiggle room for aggressive cornering.On 7s you can corner body wise, but you will need to run a higher tire pressure to avoid the tires rolling over the treads, which causes the back-end to steer the car.
So the order of things is this; choose the tire. Then the rim. Then the bs.
And as to bs; A hair too much is better than not enough, cuz with too much you can add a wheel spacer. But with not enough, you are kinda screwed, cuz the tire is into the fender.
As a side note; The checking rim for a 275 was probably also a 7.5 inch rim, making a 275 tire on an 8incher about 287mm or 11.3inch section.And on a 7incher it might be 265mm, or 10.4. Now I have run 275s on 8.5 inch rims(68Barracuda) and that works very well.They corner reasonably well at 28psi, straightline well at 24psi, and have a long treadlife at 26psi. But on 7s, those 275s will corner like crap, unless you really pump up the pressure, which will lift the outside of the tires off the pavement, and then it will hook like crap in straightline mode. And unless you run a very low tire pressure, it will burn off the centers of the treads in no time. The 50 series are worse than the 60s.
Diminsioning the tubs is pretty easy with any old rim(the wider the better) mounted on on the axle,using nothing more than a plumb-line,a sheet of posterboard taped to the floor, and a stick of wood clamped across the rim edges. But you have to know the height of the tire you plan to run.
And as a final fairwell,With street suspension, almost nothing hooks on the street without a traction aider.
