Toyota have a 6 speed Getrag - maybe someone in the States makes an adapter plate - and get an R34 GTR diff (3.545:1)
Just had a look on TradeMe and used RB25 boxes are $2k - $2500!
Have you checked to see if you can buy them brand new from Japan?
There are a couple of threads on this site with instructions and a parts list for the manual conversion - the centre console may be hard to come by as well.
A bad exhaust leak from downpipe gasket or exhaust manifold etc
or broken ring lands? Try a compression test.
Or use a long screwdriver or similar and stick one end in your ear and the other on the block, head, cambelt cover etc to try to locate the source of the sound.
No not really - its an old technology heavy under powered car - but...
Measure your fuel consumption in Litres/100km for a benchmark.
Make sure you have good coils and plugs (gap them down to 0.8mm) and that timing is correct.
If your O2 sensor is original (as is likely) replace it.
Keep your tyres up to the correct pressure
Tread lightly on the accelerator and avoid late braking
Leave the car at home and cycle to work.
I'm not saying 330 awkw is not a decent lot of power but it just seems (in my opinion) a barely adequate return for the huge amount of time and money that has gone into this (well executed) project. And of course it needs a better fuel supply. The owners seem happy with it and that's fine.
VH41 is possible. There is a thread on here somewhere about putting one in a Stagea with a Manual AWD box. Must dig out the thread and see how its going.
Not just drifters but many people modifying their cars to produce more power than the box can handle. They are in demand for manual conversions and as replacements for smaller weaker boxes. But remember they are all around 20 years old so likely due for a rebuild anyway. Not much point in paying $1500 or more for a second hand box and then having to pay out again for a rebuild down the track.
The big box RB25 DET gearboxes are very hard to come by and usually not cheap. The ugly answer is to get a GTR box and cut and shut. Probably your best bet is to bite the bullet and fix yours.