Yah, that's tight quarters. All you need is a hose barb for a 5/8 line. On the MCX, there's a 45 degree angled brass fitting and a valve. That is the place where you want your supply line.
The return line is a similar plug on your engine's circulating pump. Depending on the boat, might be kind of hard to get to. IMHO, using a Y-pipe
for the return line is easier to deal with and additionally is more effective in that it creates far better heat output at idle or low RPM.