I agree with the choice of a Nova subframe, after market steering geometry parts and pieces are readily available and same with the rear diff. Lots of after market products out there from stock to 4 link or whatever your heart desires.
As for the engine, I would stick with GM, easier mounting, easier to upgrade in the future if you set your sites on a V8 and easier get parts for.
Sorry Brian, but if he's got a Nova subframe, I'm thinking stay with a GM power plant. The car isn't original anyway.