I THINK your JC Higgins 66 was made by High Standard. Decent shotguns- but made before the steel shot era, and the harder shot will not sit well on the barrel.
Bet your has a "Dial-a-duck" adjustable choke- if so, slugs are NOT recommended in most cases- possible damage to the little "fingers" in the adjustable choke. If you just gotta, open the choke to full.
can you add rifle type sights? With enough money, you can put a flamethrower on it- but honestly, it will likely cost you more than the shotgun is worth- which is likely in the $200 range.
These are decent shotguns for the money- good shooter, not high collector values- if you want a slug gun, go get a Mossberg 500, Remington 870- or keep an eye out for a used JC Higgins Model 20 pump- it is also a High Standard- just get one with a slug barrel.