Biggest tire under a stock suspension dart????

Im runnin 275/60R15's on the back with no trimming at all.
8 3/4 rear, stock springs, 3:55 gears 400hp with a pinion snubber.
she only spins for about 5-10 feet then she goes.

:burnout:

Factory Undercoat :thumblef:
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I am running a 68 dart gt with a 8 3/4 rear end with super stock springs. I have a brand new set of 14x7 sbp cragar s/s wheels. I need to desperately find out what size tires I can run under my car without any modifications to the body, offset shackles, etc. nothing at all no modifications. the cragars are regular stock offset sbp. I have asked this on here before and got 100 different answers. I know that there is someone on here that can nail it dead on. as always I look to my mopar family for their help and understanding. there is so much knowledge here on this board that I know there is someone that can answer this. I hope I am not asking for too much. thanks brian