I've used 2 of these in bottom of my PC-60, they are 5" wide x 7" tall counting tanks. Thats with the tubing cut off flush and holes plugged. First one I epoxied 3/8"x 1/4"NPT barbs into side of tanks. Second I cut about 2" of the barbed end of tubing (5/8") flattened the ends a bit so they were oval and epoxied them to side of tank.
http://www.heatercore4u.com/2-563.htm
Here's the one with the 5/8" copper tubing, I used Oatey's Epoxy Putty, it's very strong and much easier to work with than JB Weld.
peace.
unloaded
BTW, these cost about $50 from local autoparts store.