Argh! That is one of the worst bypass pipes I've seen in a while. Neat repair & refinishing job. Watch Ebay. Good water manifolds pop up from time to time.

Watch your coolant concentration or the next problem will be the head & chaincase.

Good luck!

 

A round ball tungsten bit for a Dremel that's meant for carving also works very well.
It resembles dentist drill only bigger and acts much the same way as with rotten teeth.
It quickly gets past all the oxidized aluminum until it gets to the healthy stuff.
Works better then a steel brush as it gets into places where the steel brush can't, but the steel brush works well enough, since it's being covered in JB-Weld
JB-Weld is excellent for this sort of repair.

 

Very neat. Nice repair.