The 3.7 H150 only came out in one type of B310 wagon, this is why they are so hard to find. To be honest they are one of the hardest to get, especially at short notice.

I think they were featured in the wagon with with the squarer rear hatch and not the sloped 'faster' hatch. I can't remember the details but it was along those lines.

The higher H145 ratios were featured in the VB10 and B20. I think the only reason the B310 wagon featured such a low ratio was because it had a much bigger (torque) engine.

If you want stock you might want to start pursuing Yue Loong, Taiwan. The wagons were quite common there.

Good Luck with the search, 'want it now' and 'hard to find parts' normally don't go together.

Do you have any other ratio alternatives for the race?