i probly wouldn't waste your money on u/d pulleys... or if you really want one make sure they are dampened as they are known to flog out bearings if they are not
you could also get a new set of headers to help that exhaust flow
top speed of a supra depends... if yours is a north american model then i guess it's 130mph or something
if it's got a jdm ecu then 110mph/180kph you hit speed cut
and no you can't get a 6sp trannie for the mk3
gl with the car and enjoy the feeling of a supra owner... and head gasket blower :P