O.K. Some further info on this . . . .
I've spoken to a seller in the U.S who has done this conversion before and he says it requires no drilling to fit but I'll need my steering arm (which I have).
He has a box, an S10 pitman arm and all the bolts for £55.
Also required, maybe, is a conversion bracket (£35), although these are designed for different alterators from the one I have so I'm gonna have a go at fabbing a bracket up to suit my alternator, first.
So, total cost for this exercise will be about £70 including shipping and maybe 5 hours work.
There's NO reason why I can't swap it back and only a few additional hours work IF I decide I don't like it.
It makes sense to try the manual route first as to install power steering in the Z28 I'll need to have fabbed new P/S hoses (as the ones from the Firebird are mullered) AND I'll need to drill holes in the engine bay to take the P/S reservoir (this will also need a bracket made for it).
Long story short it works out less work and about the same outlay to go to manual steering . . . plus if I don't like it the parts I purchased are easy to sell on again, whereas the parts the required for power steering are alot less desirable, hence will be harder to sell.
There !
