I'm unfamiliar with the L gearboxes, but can tell you that a later model G or W cannot hold the power in the long run.
They seem to have a tendency to blow up <img src="/forums/images/graemlins/smile.gif" alt="" />
R series is the way to go. A R150F from a 3VZE or R151 from a 22RE-T can handle 400-500 hp easy. My chain driven T case seem to hold up just fine, even with the 7M and 2JZ. It's a original '90 gearbox and T case and they have never had problems, on road or off road.

There is a bellhouse that is a bolt on, but that's a rare one. I believe it was from a soarer with 1JZ or 2JZ and manual gearbox. (only model with a short intake shaft)
The problem with the gearboxes is that the JZ series uses a pull type clutch instead of the push type on pickups and 4runners. Due to this the input shaft lenght is 35-40mm longer than a standerd pickup/runner with 22RE or 3VZE.
This means that you cannot use a bellhouse from a 1 or 2JZ with a R manualgearbox, and bolt on a R150F or R151 from behind a 3VZE or so. Your input shaft would be way too short. The clutch disc would float in midair, that short.

If you haven't got the sourches to do custom work yourself, you can get a new bellhouse. Last time I asked it would have cost me about 800$. <img src="/forums/images/graemlins/scared.gif" alt="" />
Maybe you are lucky and they are cheaper in the US.