C-10 SBC Loss of power and running rough

1983 Chevy Silverado 305 SBC with Edelbrock Intake, Edelbrock 600cfm carb, HEI Distributor, shorty headers. I bought the truck completely stock a few years ago; it ran good, but not a powerhouse. I put the performance mods on it to beef it up a little and it ran a little better, but really no big difference. It's always been slow/sluggish from a stop, but will do 80+ on the highway without any trouble. Lately, it has lost low and high RPM/speed power, takes a long time to get up to highway speeds, and the engine seems to shake more than usual, and runs/sounds really bad. It does NOT hesitate, spit, sputter, or anything like that, it just has no power and is running very poor. Does this sound like a timing issue, a misfire, fuel issue? Thanks!