As far as the brakes go, I don't think it changing the spacing. I guess the axles would depend on the specs. Replacements made to factory length shouldn't effect anything.
I think the spacing remains the same, if you get the package from Strange, but don't quote me on this. I narrowed mine, went with new axles, and brace mine. Surprisingly, a good price.
Go here for info. http://www.bearsperformance.com/
I bought my setup from K and K Racing. http://www.kkracing.com/
Kevin at K and K can answer most questions too. Tell him I sent you. That'll be worth about a cup of coffee. LOL
Here is one from Scarebird. It's not a kit but a true converison I believe it uses GM parts. It comes with the axle plate and you provide the rest of the parts from the parts store. I have the complete insctructions if anyone is interested on how it's done.