Odd that a 1990 C30 doesn't already have a 3 inch HX
since that's what came with the M25XP engine. If you have an M25 engine, by all means go to the 3 inch HX, it's a very good enhancement. We did, cost was $270 about five years ago from a local Universal dealer, seapower, in Oakland, CA.I agree about the Seaward 6 gallon water heater, don't bother with the stainless steel model, get the galvanized one. Check out your hoses first and get the unit with the hose connections where you can get at them, and while you're at it, replace the hoses. See the link (includes pictures - scroll down) for the reasons why.