On the right rim, I'd bet the biggest 14" tire you can find will fit. Biggest 14 I know of is a 245 60 14. They would be too big for your 6" rim, however.
Gen2 front wheelwells are kinda small, for tall tires. Seems 26" tall is about the most anybody runs on the front. The rears aren't real big for tall tires, either.