Your questions about upgrades to your 79 SO REMINDS ME OF MYSELF about four decades ago, when I began my life-long PMD F Body love affair.
I'm hoping I can help you make YOUR best sensible decision.
If driving a fun, fast car that gets excellent gas mileage with the pettiest cash spent in the shortest time possible is the goal ( And for me, that's what IS important ), your 2nd gen Convertible may not be the best choice for you.
Your ride is cool for sure, but it would take a CROWD of time and money to be a "FUN" car.
Go for a ride in a 1998 thru 2002 4th gen TransAm (or Formula) LS model 6-speed.
You'll be floored at their performance, and they get decent gas mileage when driven right.
I had daily drivers of a 70 TA in College in 1985, then in 1971, 455HO was my daily driver for about two years. I LOVED them; both were fast and fun, but they were Gas-Guzzlers, and it was expensive to drive; I could NEVER do it with current gas prices.
IMHO, "Dr. Oldsmobile" would cost a small fortune to do your upgrades, and it just couldn't replicate the LS engine's performance and economy.
You don't NEED to swap out five grand for a fantastic ride that has 40-year-old technology.
My first time diving a '98 Ram Air LS Formula with a six-speed was WORLDS ABOVE both of the earlier models I drove.
Drive what you want to drive; KNOW what you are in for with the MEGA-project your '79 would be.