Brian - lead loading was very common even in seventies . It was and still is seen as a better method of body repair over fillers etc . Unless you have plenty of experience then leave it alone . Its an art form applying it in the way its needed to minimise final sanding. Its also a bugger to shape and sand.
Modern fillers are far beter than they were , as long as the area filled is well prepped it shouldnt cause any probs.

Ive just done the same repairs on my viva e coupe - the finish of lead filling wasnt perfect (camels hump comes to mind) so i ground out with heavy duty wire wheel on grinder (no blowlamp) , tidied up the welds (seam welded) and its ready for filling.
cheers - jon