I would suggest using the plastic fillers that if properly applied bond quite well and are much easier to work with.
I'm no bodywork 'Guru' Denny....but I've always said, if the body guys 60 years ago had 'bondo' available to them.
They would have probably used it.
I DO understand lead has its uses too.(In high stress areas as you mention)
I have no qualms about using 'bondo',as it's what I understand and my limited body skills allow.
Everything in moderation though, I'm trying to use as little of it as I can!
I try to work the metal to a stage where it just needs a light skim coat.
My 2cents....