Backfire/Short?

[Iowa]

My son drove my pickup last weekend and said the truck backfired
and blew the muffler...it did. Also he had to jumpstart a couple times to get home.(Alternator not charging). Thinking that was the problem I replaced alternator, wife took this morning and lights went dim after a couple miles and backfired, started chugging, she said she had to run with lights off to work. (not good idea but she had to get to work). Every time she turned lights back on it would backfire very loud and act like it wanted to die. truck is 84 chevy 3/4 ton 4x2 with 5.7 350. Anyone have any ideas? Thanks