if you are backfiring through the carb you may have an intake valve issue, either chipped, bent stem or not closing well. Intermittent could be due to something like a broken valve spring which would have you relying on compression to slap the valve shut. Broken or weak valve spring could also cause some return lash (excessive) in the whole rocker, push rod, lifter, cam lobe motion delivery.
dig?
Pull valve covers and twist on each valve spring with valve in it's closed position. This will be the spring's most relaxed position, it should offer some preload on the seated valve, holding it closed.
A broken one will be obvious, weak springs could be random here or there caused by the spring fatigue or possibly a sunken valve.
You probably got it pegged Dave...what kind of odds would you give as to the cam being boogered?
To the OP...since this is your only transportation vehicle & you seem to be on a limited budget, I'd suggest finding a runner (engine) on craigslist that you can temporarily install until you can get your engine rebuilt. Since your current engine is an unknown quantity, I don't think I'd bother trying to chase down noises...especially if they are "nasty sounding"...chances are that this engine has had more than a few "easy fixes" & is more than ready for a full rebuild. You might be amazed at what you find once you start tearing the old one apart. Find a good, reputable machine shop & talk to them as to cost...swapping out an engine shouldn't take more than a day...maybe two if you take frequent breaks...
Another thing you might consider (if you go the "beater" route) is to find one in a similar vintage & keep it as a "parts truck" for tranny, axels etc. good luck
