whats the fitting size on the coolers are you having to use reducers to adapt the line to the cooler

Cooler uses the same -10 fittings

This particular thermostat Starts to open at 160 degrees fully open at 180 degrees

thats good, my point is if there is a reduction in line size in the system you have a choke point, my comment about line size is looking at what Mercury uses on the 525 EFI now where near -10 and its around 540 - 550 hp

Of corse the fitting will be -10 or you could not connect the hoses.
What we are concerned about with the fittings is the NPT (pipe thread) size. The NTP sizing is a strange outside diameter spec.and can easily lead to choke points in the oil flow. Internal thread size of the cooler and filter head should be a minimum of 3/4.
As far as the thermostat just bite the bullet and do it right either the Hardin filter head with thermostat integrated or even cleaner discard (resale) the block adapter you have and buy the Hardin Block Adapter with integrated thermostat. Doing it right seems expensive but wait till you get it wrong because of cutting corners.
Also buy quality AN fittings and whenever possible use long radius fittings if angles are required.

Glenamy, My mistake. I wasn't thinking that the -10 might adapt to a smaller than 3/4" NPT side. Mine are all 3/4". My first real experience using AN fittings.
You don't dislike the idea of a separate thermostat correct? You're saying for a cleaner look replace the Hardin block adapter I have.
I see the Hardin thermostat is set at 212 degrees. The "Earls Thermostat" is lower. I assume that the higher temp is more in line with marine use?
Thanks again for the help.

I know for a fact that Hardin uses the stock Mercruser thermostat. If you go with the block mount you still need the filter head. If you go with filter head you can use your existing Hardin block adapter You want to organize your lines to keep the total oil line length as short as possible. If you are using a stock filter head as your earlier post suggests that is 5/8 and restricts to about 3/8 internal oil passage.
Another question, did your machinist disable/plug your block oil bypass valve?

Actually my stock filter head is 3/4" NPT. I don't see any "choking down" internally either. I am bringing them to my guy for "deburring". I can ask him then if he disabled, plugged the block bypass valve. Would that be a good thing or bad thing to disable? Mercury on their higher performance stuff come standard with an oil thermostat and it's a 212 degree unit? With oil temp being so important, why don't they have a lower setting on the thermostat.

I have a pair of oem Merc HP 500 thermostatic filter heads that I might be able to let go of if that’s what you need. Includes the brackets that bolt to the back of the left bank cylinder head.

I used Hardin oil coolers with built-in thermostats so these are just sitting on the shelf.

So this is the cooler I purchased as a kit. Whatever the "real" oil connections are, doesn't matter. ALL of my connectors are the same size. Stock remote oil filter block, cooler, and the Hardin block mounted adapter.