What would it cost to replace a V 6- 4.2 Engine in 2001 F 150 p.u.new engine or a 'Long Block?

If you need to ask, a long time. An experienced mechanic with the right gear can do it in a day. But a long block is stripped down, so you'll need a lot of gaskets, new bolts and scraping and cleaning to transfer all the old components to the new engine. It's a much harder job than swapping a used engine in.

And, it doesn't make much sense to leave the old radiator and hoses in when you're going to replace the engine.

Depends. Doing it yourself or having a shop do it? There are no "new" engines so you'll be pricing either a rebuilt or the short block you mentioned. Best to figure out what you are going to do first.