Another possibility is to look for a wheel dealer that can sell you the Panasports and that also has good wheel installation measurement tools.
Some measurement tools:
Percy's WheelRite:
http://www.jegs.com/p/Percy's/761153/10002/-1
List of calculators:
http://www.canadiandriver.com/forum/index.php?topic=55224.0
The most definitive way would be to get a scale drawing of the cross-section of the specific wheel, make a scale copy of the cross-section of the wheel in thin plywood or foam board, then lay it on the hub and rotate to check clearances. The Percy's tool approximates this. But you will at least need some detailed measurements of the wheel from Panasports.
If you use the wheel/tire industry "Plus 1", "Plus 2", etc. standard, a V8 MII with the factory 13x5.5 wheels and 70 series tires would run 14" wheels and 60 series tires in the "Plus 1" standard or 15" wheels and 50 series tires in the "Plus 2" standard.
Your choice of which tire and wheel size to use should be influenced by your plans for the car. Cruising and drag racing would indicate 60 series tires, while road racing would indicate 50 or 40 series tires, plus a larger wheel to accomodate larger brake discs.
I found an old set of 15x7 Enkei 92 clones (Fox-sized 12mm offset) that I plan to run 50 series tires on. The question yet to be answered is whether I can run 225 width all around or if I will need 205's in the front, and whether I will need spacers. Since I already have the wheels, I can mount one to check caliper and ball joint clearance, then approximate tire section with a piece of foam board.
If you are buying something pricey like Panasports, then the ~ $90 price of the Percy's tool might be worthwile.
To paraphrase Norm on "This Old House", 'measure twice, purchase once'.