you'll get loads of advice on this Fried Okra! ...I suspect that that possibly the upper or lower shaft are not clicking into place here's my suggestions
and PS the lock screw at the side of the block should not stop rotation ... it's there only to stop the dissy lifting up.
1. Install the lower distributor shaft by inserting it into the cylinder head. Oil the shaft with engine oil. Use a screw driver in to rotate the shaft so that it slots into place. The shaft can be inserted either end down as both the key ways are off set.
2. Place the distributor in the cylinder head whilst correctly positioning the locating pin with the hole on the head. Rotate the cam on the top of the shaft to rotate it so that upper and lower shaft mate correctly. (
Very occasionally[ NB THIS IS NOT THE PROBLEM IN YOUR CASE AS THE ORIGINAL DISSY FIITED OK] one may find the shaft too long if the head has been machined. Grind the shaft carefully if this is the case but check several times that the shafts, distributor and head are correctly mated in every other way are correct in every other respect before undertaking this irreversible step!)
3. Install the locking screw and jamb nut into the side of the head. Screw in the locking screw only far enough to lock the distributor in place to eliminate it lifting out. Adjusting it too tight is unnecessary and can cause binding of the shaft and bushing.