I'm thinkin bout droppin a 351 Cleveland into my 89 finder. do you think i can do it or do you think this is an extremely bad idea? why

You can do it and Yes it is a bad idea...I've talked to several people that have put extremely large engines in small places and they wished that they'd gone at it more moderatley...say a 302.

More room, you can change the plugs, in the car, and easier to work on and to get to fit....you won't have to cut the fenders, firewall, or move the radiator (I don't know if those are necessary, but they are possibilities).

If it were me, I'd go v-6...4.3 Chevy, or if you just have to have a Ford (now that is a self-explanatory extremely bad idea) then how about a 4.0 from an Exploder, or a V-8 from an Exploder.

All of those options will run better than a 3.0 V-6 and fit.