Sorry for this somewhat non-answer...the widest tire that you can fit back there is a lot wider than the widest tire that you can put on an 8" rim (which as far as I've seen a tire shop mount according to "their policy" is a 255).
But perhaps S13REMI's thread on how he put 10" wide Corvette wheels with 305/30 on his PPV will help, or at least inform:
http://www.newcaprice.com/forum/viewtopic.php?t=1055
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1987 Landau (383 stroker, vortec heads, 700r4, Bilstein shocks, Eibach springs, Bassett 15x8, BFGoodrich 275/60, 9C1 speedo)
2011 9C3 (K&N CAI, Weldcraft widened 18x9 factory steelies, Bridgestone 285/35)