Plumbing question: Removing a sink faucet assembly

I live in a mobile home which was built about 1960-61 (we’ve been here since 1987).
In recent weeks, the bathroom faucet has started to drip. We decided to replace the entire faucet assembly–spigots, handklers, base and all. It even includes a drain stopper.
When I prepared to remove the old faucet, I turned the water off (or course!!) and unattached the water pipes, by unscrewing the brass nuts with a basin wrench. But the big plastic nuts (threaded inside, with the visible part, which is round, having bumps on it supposedly making it possible to unthread them from the short pipes beneath the faucet base; these pipes are threaded and the plastic coupling nuts fasten the assembly to the sink.
Unfortuately, since the nuts have been in place for about 45 years I can’t get them loose.
How do you remove them? :mad:

45 year old plastic sink nuts will be brittle. They’ll break off if you go after them with a hammer and screwdriver. I just did this with my kitchen sink; worked like a charm.