New Rims
 

I'm considering changing out my factory 15x6.5 (225/75/R15) polycast rims for a set of magnum 500's. 15x8 with a new tire 235/75/R15 WSW (if I can find a WSW in that size)
I'm hoping this will give the car a wider and more aggressive stance without losing it's factory look too much. I know the magnum 500 were still offered as an option on the ranchero and LTD II sport models of the same era, so I'm not going to far in left field here...any thoughts or advice?

Magnum 500
 

It's good question. The 77-79 Thunderbirds were manufactured as a mid-size luxury vehicle, sitting on a Torino chassis. So, naturally, the factory wheels were appointed to enhance the luxury look. Even more luxurious were the turbine wheels, standard on Towne Landaus, Diamond Jubilees, and Heritage models. So, if you use Magnum 500's, it will be a mix of sporty and luxury combined. I've not seen an example, but I have seen other type wheels utilized to perk up the stance. I use turbines on my own 1979 model.

For the 235/75 R15 tire, a rim width of between 6 to 8" is recommended. Being on the upper limit wheel width may not be the look that you are after.

Thanks guys for the info. Much appreciated. I'm comfortable with the 15x8 rim size which will still accommodate a 225 (8.8 inch width) tire according to spec. I like the look of my existing tires (225), just wanted to go with a wider stance. The 235 mm has a 9.2 inch width.