I used JB Weld on my modified heatercore. It came w/ 5/8" fittings, but I put 1/2" hose barbs on it.
That's how it started. To be honest, i've seen leaks in H2O cooling, and it is something that I DO NOT want to deal with. JBWelding fittings on seemed the best way to go. I somehow needed to have it work w/ 1/2" hose. First i got some "sealant tape." It isn't sticky... it just wraps and then expands to form a seal. Here's a pic of that:
I could tell that that there was a quality seal. However, they were loose so I knew JB Welding to be a surefire way of having them stick

. When it dried, it looked like this.
I was a bit sloppy w/ the JB Weld juts to make 100% sure that this heatercore wouldn't leak over my dead body. So now i have the final product and i'm not sure if I see the advantage to your way. What did you mean by using hose clamps? Are you talking bout hose reducers?
-Kevin