I have a 1980 malibu that I have really modded toward Protouring. It is my favorite car because of the T-56 and being able to blast thru the gears, however I can not in good faith tell you to sink a ton of money fixing a 78-87 g-body up. The old adage applies here. How do you build a $15K g-body????? Spend $30K on it.
While these cars are building momentum, they are still a long way from being in the forefront. Be careful about how much you put in one if you ever plan to sell it. Mine will probably never be for sell so no biggy for me, but it still hurts to think I have double in it what it's worth.