The Alumacraft 1648 is a nice boat. What model is your Johnson 20hp?
I run the same boat with a 1987 Johnson 40. There is no console in mine. Too much complexity...sacrifice too much room. Before adding a ton of lumber to your boat, put the same weight in sand bags and take them for a ride. If you can live with the reduced performance, then order your lumber. You will have made a better informed decision.
If you are set on a console, there are two choices for steering. Cable pulley or Teleflex cable. Teleflex is a far neater more modern set-up. Teleflex has a website with ALL the information you need.
Use care when drilling holes in your nice new boat. I toss and turn all night long before drilling a hole in an aluminum boat. A drilled hole is permanent scar. Boats full of holes don't bring high prices. You wouldn't believe some of the nice old collectable boats I've seen ruined with saws and drills.