Everything has been covered but because I'm bored I'm gonna through my  in.

1st off,Yes there where 20r's with mechanical pumps.Those heads not cast for mechanical pumps where also not cast for a p/s pump.I think most of the one's cast for pumps came in celica's.Every original truck motor I've seen doesn't have p/s or mechanical pump.

2nd, it you want to go electric,yes you can just leave your pump on the block,no need to take it off unless it's in the way.Advance Auto parts sells an electric pump rated at 2-3 psi.Perfect for off roading with no need for a regulator.And at 9 bucks,it's a good deal.Just mount it at the tank because it will burn it up if you mount at the engine.They are designed as pushers not pullers.Just mount a filter before it and call it good.