I don't know if a 948 center cap can go on reversed but that would be my guess, followed by wrong size bearings, followed by bent crank.

Pull the caps, lift the crank out, inspect the bearing shells.  When it would not turn by hand and you then forced it to turn with a wrench, that most likely would have damaged the surface coating on one or more bearing shells, hopefully leaving you some witness marks.  Pull the shells out, read the size code on the back, verify they are right for the application.  After that, its time to go back to measuring everything.  Check the crank at least every 90 degrees and 3 places across each main and write that down, install and torque the caps on the block (no crank) one at a time and do the same for the bearings.  If you don't have the right measuring tools, take the block back to the machine shop and ask them to measure it for you.

Oh, and while holding the crank vertically with one hand, strike a counterweight with a small hammer.  It should ring like a bell with long sustain.

Hopefully your machine shop was careful when grinding the journals.  You will not find an offset grind on the center main by checking with a micrometer.  Instead, put bearing shells in only the front and rear block journals, lube and set the crank down on those two and check the center journal runout as the crank is rotated by hand.

I build some very nice 948 Sprite engines. A book could be written on the reasons your crank is tight. Steve (CTR)

Check your bearing clearance's with plastigage as well.

oops redundant.

should spin by fingers

Lots of variables here. Not the least of which is we don't know if you are using assembly lube or oil on the bearing shells when you are assembling it and whether or not you have mis-matched the caps or if you have flipped one around 180-degrees.

Make sure the caps are in the right spot and are in the right orientation. That really is quite important. Next. make sure you are using assembly lube or at the very least clean engine oil. And also, and I can't emphasize this enough, make sure the bearings are in correctly with no dirt on either the back of the front side.

You may try some plastigauge to check the bearing clearance.
https://www.youtube.com/watch?v=kLRIgaDnpds

Hi folks!
I'm rebuilding a 1959 4-door with a 948 cc engine.  I'm attempting to replace the crankshaft, and when I bolt down the end caps it seems very hard to turn.  But then, this may be normal, I'm not sure, so I'm throwing it out there to all of you.

When I first put on the crank it rotates freely by hand.  But as I tighten the end cap bolts it gets harder and harder to turn.  When they are torqued down to 60 ft/lbs, I can't move it at all by hand.  When I put on the starting handle dog, I can turn the crank with a wrench, but with much effort.  Is this normal?  How tight should the crank be, with the end cap bolts torqued down?  Should it turn as freely as when they are loose, or is it normal to expect there to be more restriction of movement when the caps are tightened down?  

Any help is greatly appreciated!

Mike in Oklahoma