Is it possible to convert a Lexus IS 300 transmission to manual?
yes, but no
technically it can be done.. but it would be very expensive.
It's not just replacing the tranny, you'll need a new starter,ignition, differential ... the list goes on.
It's cheaper to trade in your car for a manual.
PS: Why? As automatics go, the is300 has a great one. If your boosting performance you could upgrade your tranny ( or parts) to handle the increase strain. If you just want the control, get used to the paddles (buttons, with some practice they're very useful - try going into a sharp turn at 60, keep your foot on the gas, down shift , and power through)
trade in for a stick.
Sure it is possible. It will also be very expensive and require lots of modifications. It is much easier to go from manual to A/T. I would advice against it. Trade cars if you want a manual. It's possible, but if you really want to do it, I would suggest just getting the 2JZG(T)E motor. It'll cost you about $10k in parts and labor, but it's well worth it. A LOT of potential, 330 HP…