Good news, Bud.
The order of importance in the starting system (with no wasted $$ spent):
After checking out the battery and all components in the ignition system, if the engine is cranking slowly, check out (or install/service) the following:
1. Good cables from the battery - both the positive lead and the negative lead (1 gauge to 00 gauge, copper).
2. And, clean/shiny contacts for the cable-connections - clean metal, protected with conductive, anti-oxidant grease ( Kopr-Kote, Kopr-Shield
(no longer listed by Eastwood
), or NAPA: BK 7652569.
Next to consider (although, I now do this first on my trucks):
3. From the battery: run the positive cable to the starter (the normal arrangement) AND the negative cable to a starter-to-clutch-housing bolt (not the normal arrangement). Be sure to still use other frame-to-block ground cables (and maybe a head-to-firewall ground cable).
4. Then, rebuild (or have rebuilt) starter, or buy a new/rebuilt starter (NAPA now has them, as low as $60).
If the above actions still fail, you will have good starting components (that you will need anyway). Unfortunately, you'd have to look elsewhere for the cause of the slow-cranking (too tight an engine?).