St170 gearbox.

[New to this]

HI, just a few problems with my gearbox, so I recently picked up this car and the gearbox has to be one of the most shocking gearboxes I have ever had the pleasure with, it's very notchy in 1st,2nd and reverse and there is a fair amount of clunking when changing gears, I took the top of the gearbox off and there is what seems to be like metal shavings on the inside if that helps(maybe cables or something).

Also I was told of a valve being constantly open that needs fixing as when I'm driving at any speed there is a loud whistle(like a turbo kicking in)it's a box at the rear end of the car but I don't know the name of it but not sure if that would even be a gearbox related problem

Last question: if the gearbox was on the way out would there be anything stopping me from putting a single mass 5 speed gearbox in it instead of the current 6 speed dual one it has?

Thanks.