I Had a 1978 RANCHERO, the kid ate it.
I thought that I knew about it. It had the 351M.
Computer lasted 2 to 8 yrs.
TWO things got me
1 1 inch fuel filter on carb blocked, low speed ok, high speed 3 to 7 miles.
2 It had an electronic sensor (go bad, when hot),
for the firing, It took about 3 mo to find,
I ended up carrying timming lights etc, and I finely found it.
The timming sensor (breaker for the points) would die (no output),
replaced the pickup and cured he problem.
Could not find anything previous so I hopes this helps.