Fresh blocks do run hot. Just the friction of rings can add a lot of extra heat. Are you certain the ring end gaps were to spec? If those were closing up... ughhh.

This is what I do to begin:

1) Of course, check the lower hose for a spring inside to make sure it can't collapse.

2) Fill the system with a good 50/50 mix. You shouldn't need anything like additives.

3) Removing the thermostat can actually raise temps because it allows the coolant to flow through the radiator too fast. So make sure it's in there, but verify it will open if you place it in a pan of water and heat it up. I've more than once in all my 30 years (doing this) seen "dual failures" of thermostats. Don't just think "Hey, it's new, so it's good."

4) For the sake of argument you could install a stock fan, but the text Jeff posted makes sense to check regarding electric fans.

5) With all the above checked, start the engine with the cap off and run it about 5 minutes and check to see if you ever get movement in the radiator (wear a face mask in case it burps at you). As the thermostat begins to open you should start to see movement. If you never do before it boils over -- take the thermostat out and run it again. Movement then should be instant. If it's not, then you've got something wrong with that new pump... though there's not much that can fail in these.

6) If there is a air-bubble trapped it can act like it is boiling over when in fact it's just purging air. So stand read to add more water ever time it burps.

Barring any of these things being the cause, you may just have an engine from hell and it burns fire and brimstone.