get em in under as far as you can without looking it looking goofy....
this is usually a function of the frame rail width....
what usually limits frame rail width is what size fuel cell you want to stick between them.
for street use, usually you need a 10 gallon, which is around 171/2", cant remember for sure, 1/2clearance or better, (plastic tanks tend to swell and you will never get it out again), and 2" for each frame rail, if you are going 2x3 gives 22" outside measurement of frame rail.
if you are drag only car, then you can use a smaller tank, then dont be afraid to move the rails in a little more.
If you still need a big fuel tank, you will need one custom made.
we have here in the shop a 70 lemans, basicly chevelle body, with round tube frame, 17" outside on the frame rails with 18 1/2 " centers on the 4 link brackets.
put the wheels on the tires you want to use, stick em up under the car were you want them then work from there.
I see in your other post you are kind of doing that. It is a good method, but make sure you check all measurements 2 or 3 times and write everything down., just some tips here.....
also, chassis engineering makes a wider tub than the usual 23 1/2" ones if you need, you would not need to add in filler pieces..