Paul,
I feel your pain. I went through much of what I think is happening here on a 1967 Kenmore 70 in the early 90s when I was still teething on the mechanicals of these machines.
My guess is that the spin-tube is mis-aligned in the bearings, and is causing a "pinch" so to speak. This will result in the drag on system during spin. This is not unusual in standard centerpost machines after work like this. If I was faced with your situation, here is what I would do:
1) Completely relieve the tension on the belt by loosing the motor and bringing it to the far right of the travel path in the baseplate. (By the way, you didn't have to take the motor mount apart that way - the two mounting brackets come out very easily, and both should be loosened anyway to properly adjust the belt). I always take a motor out with the brackets on.
2) Remove the agitator, and remove the basket from the drive block. This will greatly improve your chances or bearing realignment when the weight of the basket is not working against you.
3) Remove the transmission braces and lower the three transmission mounting bolts a good 6 or 7mm, at least.
4) Stand the washer upright. Then bounce the transmission up and down in the centerpost with a little force from under it. If you need more room from the bolts, lower them a little more. This should release any bind on the bearings.
5) Sometimes, just to be sure, I will grab both sides of the baseplate and shake it good - this will help the bearings and spin tube settle. I know it sounds stupid, but I do this routinely. That '67 was a major b---h.
6) Retighten the three trans bolts in a random pattern similar to mounting a vehicle tire. Raise each one on a couple turns then raise the next. Raising one too much will cause another bind. Do this all with the machine upright, by the way...
7) With the gearcase tightened, put re-mount the basket.
8) Put the belt loosely back on the motor and slide the motor to the left with a lever in between the two mounting brackets. Tighten it only to the point of non-slip tension on the belt.
9) Manually engage spin by holding the left plunger up and rotate the drive pulley on the gearcase. See if you can feel excessive drag after spin engages. There is always some added drag, but you should still be able to rotate the belt, and the drag should be smooth and steady.
10) If you are comfortable with step 9's results, properly tighten the belt and put the rest of the machine back together (don't bother with the braces yet until you are satisfied with the machine). Run a operational test. If you like the results, then you can re-install the braces.
The point here Paul is to loosen tension in the basket drive / spin tube / bearings so the parts can move freely. The best way to do that is get the weight off the spin tube from above, and the tension from below which comes in the form of a tight belt and the gearcase bolts. Shaking it all helps things settle when they are not under tension. Hopefully that will work.
I'll keep my fingers crossed!!!
Gordon