[Argess]

Sonic testing is to check for cyl wall thickness. Normal max overbore for a 427 is 0.030". I think 427's were originally manufactured to have consistent cyl wall thickness, but other FEs were not done the same way. So a sonic test makes sure that the cylinder walls are thick enough to take the required overbore, especially in other FEs where core shift may have left cylinde walls thinner on one side than another..

EDIT: rust scale will be removed during hot tanking. Best to do your other tests after that.