That is quite the story. I am not quite sure how over heating cracked the block unless it got super hot. Normally overheating does things like blow head gaskets, warp heads or seize engines.
Lets take a moment and look at this situation holistically and try and give you some things to look at/think about.
First off, the cooling system is just that, a system. Everything must work together and be compatible in order for it to function correctly or better said adequately.
- The radiator must be able to dissipate enough heat to cool the engine, this is done by overall size, core size/number and manufactured materials (brass, aluminum). The radiator size typically matches what the car was designed to use (22 or 26" for a Mopar). The core size (internal tubes) are selected by the manufacturer to accomplish a specific task. You will see 2, 3 and 4 core radiators, however some manufacturers use less (like 2) cores or tubes but they are big in order to accomplish the task. There is a lot of debate/argument about which is better (more cores or bigger core) but to be honest, they can do the same thing in many cases. In general, a large tube aluminum radiator will cool a big engine (all things considered).
- The radiator cap; this small yet important component must be correct for the application. It must relieve pressure in the system when needed. Too light and the car will puke water/coolant too quickly, too heavy and it will allow the car to over heat and damage the engine.
- radiator hoses much be rigid enough so that they do not collapse (due to the sucking of the water pump and heat) when being used.
- The water pump should be adequate to move coolant through-out the system in adequate volume to meet the demands of the system. A factory BB water pump is pretty good, the high flow after market pump can be good but they are some that are "drag race only" and really are only effective at high RPM/speed. In general, a good Milidon high flow pump or even a factory pump will get it done.
- The T stat, this must open and close as designed at the right temp in order to regulate the engine temp. These are pretty simple and available with even simple parts store samples working very well. That said, I always drill a 1/8 hole in the rim of my T stat to allow air to make its way out of the system. I really like "fail safe" thermostats because if they die they do so in the open position verses the closed position as normal T stats do
- Your coolant; you need to be using proper coolant and not just tap water (common problem with old hot rodders). straight water is fine in a race car but not a street car. First off, straight water does not have the anti corrosion properties that anti freeze has and therefore will cause/support rusting inside the water jackets. Additionally, antifreeze helps with heat dissipation during operation. You can also use an additive such as "water wetter" to further help with dissipation.
- your fan must be large enough to move enough air to cool or remove heat from the radiator. It needs to cover as much of the radiator as possible to pull air across as much of the radiator as possible. Of course there is a limit to this size and that is where the shroud comes in (more on that in a sec). If using an electric fan/s, they need to be large enough, spin fast enough and come on at the right time to effectively remove heat from the radiator. If using a mechanical fan, it needs to be large enough to pull air through the radiator. A clutch fan is fine and they work good because they have a thermo spring in them that essentially expands and it gets hot and locks the fan up more (making it turn more) as it gets hot.
- your shroud much be of a proper design to "funnel" air being pulled by the fan. It should be sealed around the sides to a degree so air cannot escape and over or very near the fan so that the fan can create the vortex effect.
Some other things to consider;
Your compression ratio - if you are running a high CR motor, they require a very big and effective cooling system because they generate a lot of heat quickly. Racers use things like water wetter to assist in this effort.
Your timing - if you are running too much advance the engine will make a lot more heat and overcome your cooling system
Water jackets - depending on where your "replacement" block came from, if there are any obstructions in the water jacket it will cause the engine to overheat. There is really no real way to know this as you cannot see into the water jackets without pulling out all of the freeze plugs and inspecting them. Since this is a new engine, I would hope that it was flushed at some point before being built.
Your pulley system - as stated, you can spin things too fast or too slow but if you bought a kit I would no think this would be an issue unless you bought something designed for race cars only.
The area in front of the radiator - if there are a lot of things or things blocking the front of radiator, that can have an effect but unless you have cardboard or something closing off the radiator, this should not be an issue.
Cooling issues can be very frustrating as you are finding out and some engines have more issues than others but based on your story something else is going on here given that the first engine failed. You have some major component not functioning correctly such as the radiator or perhaps the water pump. I will say this, there are water pumps designed for Marine applications that are designed to run backwards, thus they do not work in cars. Hard to believe you would have one of those but you never know.
This issue needs to be diagnosed methodically and addressed smartly rather than just throwing parts at it. Based purely on what you are saying, my "guess" would be bad radiator, bad water pump, or a combination of things.