Is this a good idea?

Go for it! You don't need to use the "Abody big block headers". Just use the HP manifolds - which work much easier in the low deck 383 in an A body. Yes, you can keep the power steering. Also, you can keep a column shift if you like.
This is my exact plan for my '75 Scamp as I get the bucks together. I have a 68 383 on a stand along with a 727. I already replaced the 7.25 with an 8.75 rear.