I have a new cooktop 30A (replacing old 30A) with 2 black wires and 1 green and that's it! Does it not matter which black wire goes to red??
Plus the plastic whip is bigger than any holes in the old junction box. It has a huge removable plug on it but it's still too big. Do I have to buy some adapter to get it to clamp safely into the opening?
It's a Kuppersbush.

It doesn't matter, Black/Black, Red/Red, Black/Red, and either hot leg from breaker can go to either color. You just need the 240 volts to the 2 black.
They Make reducing wasers to reduce the size of the hole to allow smaller connectors.

I have only seen and used these:
http://www.garvinindustries.com/Fasteners/Washers/Reducing-Washers

These, I just now found, and there are times you need a side clipped off, these are clipped in a hex pattern:
http://www.globalindustrial.com/g/electrical/conduit-fittings/bushings-washers/raco-steel-reducing-washers

Thanks stratmando

Anytime.