I like the Cragers S/S far better than Magnum's myself
vintage & modern at the same time
more than the Magnums
It's not my car either
But since you
asked of our opinions 
I'd do 15" if you changing wheels & tires both
(
some 17" or 18" I like too, as long as they have decent taller sidewalls on the tires
too much bigger than that, of course unless you have a Pro-touring, big brakes
& needing more rims size etc. or one off Resto-mod vibe, it doesn't look as good IMO)
there's a few wheels & tire combos I like
on older cars I like something more retro
here's a couple
Like the Halibrand look, replicas Rocket makes some
so do a couple others
kidney slots polished of course, these on my car aren't available anymore
they are Halibrand Knockoffs/reproductions from Edelbrock back in 2005-ish
these are on 7"x15's 245/60/R15 frt & 9"x15's 295/50/R15 rear BFG's
front needs to be a tad lower, but roads up here are pretty well pitted
or something like these Rocket Racing wheels
Eliminator or Igniter (?) polished
old style Keystones, someone still makes them
Crager Prostreet, with spinner caps
Bonneville Salt Flat style American Racing "Polished"
or AR Salt Flat as cast or painted center
or these flat spokes also Cragers
American Racing Torque Trust too
as long as they aren't the D's rounded spokes
many of the above wheels can be had in polished or a combo of styles
Magnum 500's look OK too,
Road Wheels, especially those without trim rings
& not the Ford Style 500 Wheels
BUT;
It's just not my thing (it's not my car either)
Now
if it was an OE car or more of that look, yeah go with the 500's
JMHFO
good luck
