I have a 1985 Pontiac Firebird sbc 350 and I want to put a t5 behind it I have found a t5 with a 10 spline output shaft is there a clutch assembly that would work with the 350

 

10 spline input would be a V6 trans, or a Mustang. Might as well just kick yourself in the nuts now, rather than trying to make that trans work. It might work briefly, but they're borderline junk when it's a V8 T5 behind a 305. A V6 T5 behind a 350, you're just asking for trouble. The way you'd do it, is a V8 bellhousing with a custom clutch disc, assuming it's a V6 thirdgen trans. If it's a Mustang trans or S10 trans, the shifter will be in the wrong place and there won't be a torque arm mount. Just aw hell naw to that entire idea.