Did the old rad fail because the plastic Hose outlet cracked?
If so, this could be another example of the famous "cooling-system-alkali-embrittlement"
Some say that the type of plastic used in our cooling systems get brittle if exposed to fluid which has any other PH than 7.0 (neutral).
There's lots of circumstantial evidence to support this.
I guess the advice now that you've changed the rad is to check the PH of the fluid and add anti-freeze untill it sits at 7.0.
As far as how tight to do up the jubilee clip..............do it no tighter than required to stop a leak. Any more will stress ths plastic.
'73 Jensen Interceptor (Resting)
Giant Full Sus Mountain Bike