Advice on Testarossa wheels

I am planning to put Mythos wheels on my Tamiya Testarossa (17 in), but after some dry fitting, i find that i need to widen the body by at least 4mm on each side at the rear, making it an overall width of 88mm from the stock 80mm. Narrowing the track seems impossible as the rear wheel wells are just tight enough for the suspension and wheels at the normal position. The front is quite ok though.
Any advice from the experts? Thanks.

Save yourself a lot of work and take a razor saw and narrow the tire and wheel to the width you need. If you section the tire in the grooves, you can piece it together so that it looks like it was made that way. Or narrow the Mythos wheel and put the Rossa tire on it. I would have thought the Mythos wheels would go under the Rossa without modification, but I've never compared mine.:)

That's what i thought at first before buying the kit. Anyway they are on the same chassis. But the Mythos is much wider than the Testarossa.
Well, let's see if there's any other advices, or else, i'll just build my first bodykit