Can you swap a five speed 01 cobra for a 6 speed?

The input shaft is 26 spline and output is 31 on the t56 from American muscle. So would that still fit in my car if the current tranny is only a 10 spline? If all else fails I'll just stick with a 5 speed but I think six speeds are sweet. especially since I want to supercharge it eventually. What do you think the grinding could be? Generally only happens when I down shift but sometimes it'll grind upshifting into fourth as well. Fourth is the primary gear that grinds whether it be upshift or down shift

Your grinding is most probably caused by worn out syncros or possibly even a clutch that is not fully releasing. Is it harder than usual to get in gear from neutral with the clutch fully pressed in? My experience with bad syncros is that they don't all go at once and it was the higher gears first. Bad clutch release will effect all gears. Do much power shifting??

No it's pretty easy to get into gear in all the gears. I have a short throw shifter on it and shifts like butter except fourth sometimes. Most the time it's smooth but there are times that it'll grind a little and then it goes right into gear when upshifting. It's usually at higher rpms I've noticed. The clutch is fine in my opinion I just got a new clutch cable and got it adjusted and it feels great. I usually push it all the way in when I'm shifting but I'm starting to just push it in just far enough to where it engages and then letting out. I don't powershift at all because I have no need to. I'm not used to the car yet so I haven't been to reckless with it haha. The previous owner might have been beating the **** out of it though. The car has 90,000 miles on it already so I might be do for a new clutch eventually but right now it feels fine. I was going to put a new clutch and flywheel in when I either replaced the tranny or got the synchros fixed and all that other stuff replaced