best carberator for the stock slant 6

For stock heads and exhaust, the best bang for buck is probably changing to the Super Six 2-barrel intake (ca 80's Aspen, Volare) w/ the Carter 2 barrel BBD carb. Many posts on that. Maybe $150 for parts.

You might do a bit better w/ a small 4-barrel carb, but intakes for that are $200+ and only helps with a custom head job to bigger valves, bigger exhaust, and maybe bigger cam. Maybe $700.

By far the best performance is to design in a turbocharger. That is high-skill work, but can still be affordable if you are resourceful, can weld, and adapt junkyard parts. Search posts on that.