bushwood

I have 440s on my boat and performing maintenance on the cooling system. I want to replace the thermostat while I have system drained and apart. However, for whatever reason I am having trouble finding a 160 degree thermostat for a big block. The thermostat that was in there was something the past mechanic put in but the engine always ran a little warmer than the other engine.

Any recommendations on where to go? I'm afraid if I go to my local parts shop, I will get a less knowledgeable person and be back at square one.
Part number ?

Thanks!