Re: 69 conv wheels 18" and 20"
In the front 235/40-18s on 18x8 with 4.75 backspace.
In the rear 255/35-20s on 20x8.5s with 5.25 backspace.
The rear will work with no problem at all even on a very low car (top of the wheel at the lip of the fender. In the front if the car is very low and you want tire sidewalls to be equal front and rear you can drop down to a 225 tire.
There are plenty of cars here with wider tires in the front and wider wheels and tires in the rear. When it comes to the really big wheels in the rear the car has to sit pretty low for them to look good (althought some people feel they will never look good LOL) You may be able to squeeze a 275/35-20 on a 9.5 wheel with 5.5 to 5.75 backspace in the rear but if the car is low it will be very tight and will probably rub on curb cuts. The sizes that I gave you at the beginning of the post I have seen work on a 69 so I know they are good.