wheel cluster... need a little help

[topdown99]

ok, so long-story very short, I need some help with wheel backspacing.

I have now purchased 3 (THREE!!!) sets of wheels... and none of them fit... so I need to sell 3 sets of wheels.

first set the backspacing was wrong - the tires hit the steering arm
second set was a 6-lug set - I am mid upgrade to 5-lug so they are useless to me now
third set I flat measured wrong... got excited about some inexpensive wheel vintique chrome reverse and measured the bolt patterns the same way I measured the 6-lug pattern and thats incorrect. so its a 5x5 and a 5x5.5 wheel not a 5x4.75 and a 5x5 as I thought.

So... now I have a chance to buy some corvette rally wheels that are 5x4.75 but I dont want to re-do my backspacing problem.

1957, i-beam 3" drop axle, stock steering, disc brakes, 5x4.75 - what backspacing do I need to get the wheels to fill the wells nicely but not hit steering or frame?

Thanks!!!