Running rough and stumbling

[Dmax21]

I have a k20 that's been stumbling pretty bad when I get to around 60mph and yesterday it started doing it at any mph and I barely made it home, but the truck starts and idles fine and revs pretty good, it seems like the problem is while under load. The truck has a 350 with a Holley carb and been updated with a hei distributor and I recently just changed the plugs and plug wires. I've only owned the truck for about 2 months. What do you guys suggest I start with or maybe someone knows exactly what it is.