First: do as truckernix describes (and, also clean/set the removed plugs - and check/set ignition-points - and clean the rotor tip).
Then: might as well go back to step 1, and step 2, and step 3 (checking "electricals" first):
1. Check the condition of all battery and ground cables/straps
- good/clean cables/sheathing and ground-straps (no corrosion showing from within and covers/sheathing)
- correct gauge (00 to 2 gauge) for both the cable (battery to starter) & the ground straps (battery to frame, and frame to clutch-housing or starter bolt)
2. Check/clean/protect all battery-leads/feeds connection-points (coat with a copper-impregnated anti-corrosion grease)
- battery to starter connection for "hot" lead (negative for "power-lead" is preferable on a positive ground USA GMC)
- all ground-leads connection points (strapd/cables from battery to frame, and frame to clutch-housing or starter bolt)
3. Connect the battery leads (preferably for positive ground system)
- negative battery "hot" lead to the starter "power-in" fitting/connection-bolt
- positive ground battery ground-lead/straps
(battery to frame, and frame to clutch-housing or starter bolt; or, battery to starter bolt))
Crank using a good battery (6v preferred - 12v OK for short cranks)
If the engine cranks poorly/slowly, and the battery is good: the next suspect a probably a tight engine (or, an old/worn-out starter).
Other likely culprits:
- bad carburetor (with old/unclean gas)
- bad valve lash/gap settings
