I'm not sure there is a tried and true way to paint an engine. I've only painted a couple with anything other than engine paint spray bombs. Mechanics have always said too much paint will make a motor retain heat. I'm a body man/upholsterer so I always try things on my own stuff to see if it will work.
The big thing is getting the block and heads as clean as possible. If it has been rebuilt it was probably cooked to clean it inside and out which is good. There will be some oil and grease on it from going together, but that will clean off easily.
I started by buying a bunch of different sized wire brushes to go in my die grinders and cleaned and brushed every nook and cranny on the block with the heads and water pump on. Don't grind, smooth, and fill all the imperfections as this will also make it retain heat.
Once happy with the cleaning, spray it down good with a good wax/tar/grease solvent wiping and blowing off the excess. Remask for the painting.
If you are painting a good coverage type of topcoat color, one coat of an epoxy primer would be best on the bare steel, followed by two coats of a single stage urethane or acrylic enamel hardened topcoat color. This should last for years.
That's not what I did on this one. Cleaned good like I said. Then two coats of a gold 2K urethane sealer, two coats of my very transparent gold pearl base, and two coats of hardened urethane clear.
That "pagon gold" was a tri coat made famous by House of Color and some others years ago. My version is a mixture of PPG mixing gold pearl (70%) and a mixing yellow (30%). The trick is the goldish/tan sealer underneath it and then it is clearcoated. If anyone's interested I'll give you the #'s.