have a pair of HP500's in my 38 scrab excell. 3500 rpms + my oil temp will move from 250 to 275 and set the alarm off. Has oil cooler, made sure its clear, changed oil temp senders both engines, changed both guages. I did use oil fliter mounts off of a 420 with oil lines and senders.
Any ideas?

Maybe the 420s are to small a line and don't allow enough flow

Make sure you didn't cross the lines from the remote filter to the cooler/block, thus running the oil through bypass, thus not getting filtered/cooled. Did you feel the filter to see if it was hot?

whats oil pressure and water pressure? if you crossed lines you shouldnt have any oil pressure

I dont have water pressure guages. My oil pressure 60 psi. This oil temp thing has been with me a long time. As long as you sloww the boat down it will cool to 200 degrees, but anything over 3000 it starts to heat up slowly. The more rpms the more heat

Seems like low water flow through the cooler if its been a long time issue.