I would buy the 164 in a minute. Where is it?

There has to be a smashed 164 somewhere.

Also, the blown engine might be able to be rebuilt.

What would it take?--more effort than it would be worth. I don't think the B20 will mount to the B30 bellhousing (and if it did it would mean fabricating new front mounting points) - or conversely if you are installing an M40/41 or BW35 with the B20 mounted to the front crossmember a new, longer front driveshaft section will need to be made and balanced and if stick shift, clutch cable and shift linkage worked out. All in all, not such a great idea. You're certainly right about it sure to be "sluggish" -- lugging around more weight (164- longer wheelbase) with less torque and a rear axle ratio made for the 6 cylinder.- Dave

At a minimum I would want to know if the spacing between the front motor mounts and the engine/bell housing mating surface are the same between the two engines. If not then the job just got a bit more involved.

If you have a tachometer it won’t read right.

Would you be mating the B20 to an auto trans?