there are some clues there that the timing chain has jumped. its a
1974 so its old maybe by now the timing chain is stretched and it has
even jumped a cog. he had to adjust the timing to get through the
mountains. If the valves are not opening and closing in sync with with
the pistons going up and down it may or may not help to adjust the
timing. i think the confirming test is to check the compression. if
they are even and low the timing chain has jumped. if some are good
then its not the timing chain. It takes all day to replace the timing
chain (and of course you will also replace the gears)
so you want to make sure before you tear it apart.

If the engine backfires, the timing is off.  If the spark timing is okay,
the valve timing is not.  You've played with the spark timing, now it's
time to check the valves.

Might be good first to pull the chain cover and make sure it's lined up
nicely.  I might want to do a compression test before anything else,
though.  I _hope_ you did a compression test before buying, though.
--scott