Here's how you measure a bolt pattern:
http://www.mrt-wheels.com/boltpattern.html
As for your wheel, the ones that you need to look at are sold by LMC truck. Sure, you can use multiple Ford truck wheels from later models, BUT you won't have the "innie" knob design to accomodate the original "dog dish" style hubcaps if you want them. As I recall, the wheels offered in the LMC catalog have the correct "innie" knob pattern. The original F-1 wheels were actually 16 inch, but a LOT of people go with the 15 inch wheels nowadays for ride comfort and the wider bead width.