Hey Ryan, if you want a low cog, why not use an apposed engine, like a Subraru/Porsche/VW engine? If you are going to make a custom bellhousing, then you can do that with anything as long as you can get a clutch that will work with the tranny input.
And, if you do go with the Previa van engine, then FOR SURE you are going to use the Supercharged version, right? Its the Previa SC model with the Toyota SC14 supercharger unit, 1400cc. I converted from my SC12 unit (1200cc) on my AE92 4A-GZE superchraged Levin engine that I swapped into my MR2 with a SC14 unit from a 1G-GZE Jap-spec Toyota Soarer engine.
On my 1.6l 4A engine, using a crank pulley that was even 5mm smaller (less boost pulley), when I converted over my peak boost went from 13 psi to 17.5 psi just because of that larger displacement supercharger. So that previa engine has got some potential with that huge supercharger on it. You could eaisly change the pulley ratios and get over 15psi Im sure, if you wanted to..
BigMike