Want to build a 77 360 suggestions please

I’m not sure what the condition of this engine is in, I was told
it was rebuilt 12,000 miles ago, not sure looking at the paint on the motor not sure that really happened or not.
The motor right now sputters and misfires when I get on it, checked the timing its at 12 degrees ideling at 900 rpm ??
I would like to get into low 13s with this car
Don't need 400 hp to go high 12's/low 13's. A-body with 3.43 gears will get there with what ya have. Just need to really work with it.