I recently build the B 4.1 kit, and even tightening the screws as loose as you could possibly want them, the transmission was still dragging noticeably, and I did not trust with the amount of screw engagement with the motor plate they would stay secure.

Looking at it more I noticed that having left side of the transmission case off, that the gear on the top shaft did not line up horizontally with the idler/diff gear. All bearings were seated correctly and everything assembled correctly.

After more investigation, I found the culprit was the silver aluminum sleeve used to space the top-shaft. It was two long. So, I carefully sanded it down with sand-paper, may have removed about 1/32 to 1/64 of and inch. Just went little by little until I not longer got binding for reasonable snugging of the transnmission housing bolts

Also, I did not use the washers for the transmission bolts. You don't tighten them that much anyways, and I felt the depth of thread engagement was no good with them.

After these mods, the transmission runs nice and smooth.