Yes .. Also sometimes the dumps in the tails are to small, where someone modified them or they were just made that way. Its all about radius times pie squared... What goes in must be able to go out easier

I read all your post thank you, I don't really understand when using crossover with a bypass thermostatically controlled like CP 620-300100 with a big bypass hose how it's it different than a circulating pump.
When it idles your creating 3or4 psi and then when it revs you have more psi , as long if you don't get to much psi I think you good?
I know that merc doesn't make circulating pumps for nothing , I still not convinced before I buy one.
Can anyone share there experience before I buy one

If you want call me (920-475-0852) I can explain both ways in better detail. I have done several motors with both circ pumps and cross overs. There is advantages, and disadvantages to both ways. Its hard without going into pages of writing to explain.

Whats the block water pressure ?