Overheating on run stand

Just asking; you got a direct drive 7-blade fan on it for break-in, in a shroud?

1) Easiest way to get the right timing is to leave the timing lite in the toolbox, reconnect the VA to manifold vacuum, then with the rpm set to 2000, just advance the distributor until the rpm quits rising. If the rpm gets too high, crank it down to where you want it, then either RETARD or advance the timing, until the rpm again reaches max, whichever direction it needs to go. The goal is to get the rpm to rise to a peak, but not begin to drop.
Let the numbers be what they will be; it doesn't matter.

2) At the same time, there should be a temp difference between the hottest part on the stat house, and the bottom of the rad near the WP pick-up, of about 30 degrees. Twenty is not Enough.

3) If you still have overheat, with the pump known to be running in the right direction, and the belt NOT slipping, clamp the bypass hose to force all the water in the block to the stat house, instead of going round and and round inside the block!

4) If, after all that, you still have an overheat situation,
go back to your build sheet and check the skirt-clearance, the ring gaps, and the piston type.
With KB hypers, I followed the directions exactly, and had an overheat that could not be cured. I took the engine apart and increased the ring-factor from 065 to 080, put it back together, and Shazam, the engine ran cool as as a cucumber.
In fact, I had to install a hotter stat, just to get the heat up to where I wanted it.
5) Oh and BTW
At the new ring gaps,
the engine idled down to 550 in gear and pulling itself with a starter gear of 9.44 (manual trans), and with a 292/292/108 cam, and @5* ignition advance ....... with no temperature increase whatsoever.
Whereas in the earlier configuration, there was no way it would run on that tune nor would it even suffer idling at all; the car had to be moving at all times.
I learned a lot in that frustration, and, altho at the time I was not a happy camper, the experience was well worth the learning.