Yeah, something around that size would be possible if they made such a creature. Basic street treads are kind of limited in available 15" sizes with 295 being most common. If you want to step up to racing style tires, there is a much bigger selection in 15" or moving in to 17 and 18" rim sizes, there are quite a few more choices in the 300+mm range with 305, 315, and 335 being real common.
E body leaf springs are splayed wider in the rear than in the front, so this becomes a limiting factor if you don't want to perform any modifications. Depending on how much sidewall bulge your tire has, 4.75 to 5 inches may be about the most back space you can put under there without moving springs or changing housings. Dunno for sure, I've never put a wheel greater than this under there, but I think some of the 17/18" crowd use 5.5" back space since they have very little sidewall. I've got a set of 15x10, 5" back space with 295 tires that I am going to test fit under my Challenger this summer. Currently I have 295s on 15x8, but this narrow of a rim creates considerable sidewall bulge and tread face distortion. I have to run them around 26 psi to get full tread width contact with the ground, which is why I want to move up to the 10" rim. However, I'm not opposed to relocating springs or changing housing widths to get enough room.
I'm not sure about the size of an L60, but I'd venture a guess to say it is between 275 and 295mm. I think the problem with fitting L60s in the wheel wells in the early days was the desire of many guys to have "deep dish" mags that only had 2 or 3" of back space. This really hung the tires outside the fender lip
Steel 15x8 magnums generally come with 4.5" of back space and 15x10 with 5", but I think you could custom order them with something else. You'd have to talk to someone at Wheel Vintiques about it. I know the billet Magnums can be ordered in any backspacing you want and are available in 15 or 17 inch sizes. Here is my 295 on an 8" rim with 4.75 back space
