some more info would help - tho it's safe to assume that you have a 7mge engine.
model number of cressida?
model of gearbox (see code on build plate of car on firewall)?
you'll probably want a W57 or W58 gearbox, plus M series bellhousing, plus allthe bits missing from your car that manuals have.
also, search for 'manual conversion' and you'll find out more about converting to a manual box.
also, search thru the member's rides for similar cars & manual conversions.
as for performance tips - search for 'performance' in technical articles