Ok Guys.... I'm giving up one of my sources.... nose around this site and maybe you can find what you need... these are the people that manufacture coolers. http://www.lencocoolers.com/lencocoolers.html
Don't just look under Mercruiser.... look under all of the types they have under "covington" there are lots of tandem coolers... etc.

Great source! Thanks1

OK guys... here is another source.....go to the marine division....or go back to the home page and at the bottom you will see a link to the downloadable catalog.... you will find lots of goodies to look at. You really didn't think Teague Marine built oil coolers did you ?
http://www.sen-dure.com/index.html

Monty,
I am in the same dilemma as you are. I too, am building a 540 very similar to yours.
What style crossover are you going to use? I am considering the style that has a bypass to the T-Stat housing. This should help balance out the temps and help control the water pressure.
I am also in search of an Oil T-Stat/filter mount combination. Eickert has one and so does Mercury. Can't seem to find the Mercury Part Number.
Mbam told me that I had to buy the components, part by part, that makeup the one that was used on the Bulldog. I also see that Hardin Marine offers one, too.

Does anyone know the size of the HP500 oil cooler?

My present cooler is 2.125" x 14". I used it on my blower motor and it was too efficient. I had the Perma-Cool Oil T-Stat incorporated.
I had to run the boat pretty aggressively to get some temperature. I would consider running the oil flow in the same direction as the cooling water flow. This is in line as to what Gary said above but opposite.

Here is a link to Billet's one piece oil filter/T-stat.

http://www.billetmarine.com/Our%20Products.htm