Hardblok will help support the cylinders and keep them round under severe stess. It will drastically improve ring seal under severe conditions, and it's extra mass will help dampen major vibrations.
I would fill it to the bottom of the water pump holes to allow for adequate cooling.
I filled mine with HardBlok, as this was the brand that was overwhelmingly recommended over any other... But, I haven't got to run it yet.
Fill the block BEFORE machine work is done. The Moroso brand is supposed to expand as it cures, and will make the cylinders tight. I have heard of people using the HardBlok product AFTER machine work was done without incident, but wouldn't recommend that if it can be avoided.
You do ONE bank at a time. It's a good idea to torque the mains and use a level to get the engine completely level front to back and side to side.
You then mix it up and pour it in. A mixer attached to a drill will make the mixing more effective and faster, and give you more 'pour' time before it starts to set up. An old alternator fan (or even a straight bar) welded to a rod works great for this.
Be aware that each bank will take DIFFERENT amounts of mix to fill.
When finished filling, I used an air hammer set on LOW to help with settling. I welded a fairly thick 1" square piece of steel to the end of one of the chisels to act as my 'blunt' tip. This makes the job go WAY easier. Settling takes place quickly and efficiently.
After it was all settled, I installed a head (and gasket) and let it cure for 24hrs before doing the other side. The head will simulate the loads on the bore when the engine is assembled. (same principle as 'torque plate' boring/honing)
The directions in the HardBlok containor are good, and fairly easy to follow.
If you have a GOOD scale (for reading low weights of less than 10lbs) the 'tall fill' containor is twice as big and will fill two blocks. (comes with two large bags) It's just important to weigh the filler out accurately to the 1/4 pound. The 'half fill' Hardblok is already pre-measured for half filling. (two small bags)
I also cleaned out the water jackets before beginning by filling them with muriatic acid. I let the un-diluted acid sit in there for a couple minutes in each bank before dumping it out and rinsing GOOD with water. It will eat ALL the rust out of the jackets and give a great surface for the Hardblok to bond to. Just be careful. When the acid hits a concrete driveway, it produces a nasty 'acid fog'. As a by product, the driveway will have one VERY clean spot. Wear gloves and eye protection, and try to avoid getting the acid on your clothes. Muriatic acid is found in hardware stores, and is normally used to lower the pH in swimming pools and clean bricks and masonry. (it's usually diluted for this)
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3