Check out my thread here on this very topic (about to do an update in the next hour or so:
https://www.fordbarn.com/forum/showthread.php?t=226656
I have Thermocure, a product of Evaporust in my block currently (about 72 hours so far). It took about 1-1/4 gallons to fill the block to the top of the water outlet. The 32 oz bottle I bought is good for up to 3 gallons, so my dilution is about double strength. It was about $19 on Amazon with free shipping.
Warning! Science ahead:
Original Evaporust (and probably their Thermocure product) works because of an ionic process called "chelation" (pronounced key-LAY-shun). Basically it bonds electrically with the rust molecules (iron oxide) -- but doesn't bother raw iron, steel or other metals.
Molasses also removes rust through a similar chelation process. It works well, but it's as slow as... well, molasses. It might take 1-2 weeks where Evaporust will probably work in a few days. (And you wouldn't want to put molasses in an engine).
I've used regular Evaporust a lot on rusty parts in the past and have had good success, and so far I'm pleased with Thermocure.
Leaving rusty metal in either one of the products for too long won't hurt anything -- once all of the rust is dissolved, the chemical process stops. You do need to flush/rinse the metal thoroughly when you're done -- but don't throw away your used Evaporust. If you let the rust settle to the bottom, you can use it again several more times until it loses its effectiveness.
Note: I'm not a scientist, but I am always curious about how things work, so I looked this up some time ago.
Hope that helps!