I have a twin engine boat powered by Twin Cummings 6BTA's 330Hp, all stock.
I would like to add boost gauges and pyrometer.
What boost pressure range should I be looking for?
What EGT range should I be looking for?

Take a look over on the Seaboard marine website I think it’s Sbmar.com they have a combo boost pyro gauge made for Cummins engines. My brother bought one for his Cummins powered 26 Shamrock

It's "Cummins", no G in there. That may improve your search results. I've done some work for Cummins headquarters, and they get VERY mad when you add the G.

Westach

have this in my 1999 Dodge Cummins trouble free for 20 years. I have the 50 psi one but my wastegate is modified.