if you want to go decently fast, you need a large downpipe & big pipe to the back of the car. Three/3 inch" pipeing. ( Minimum 3") Do not skimp out here & dont let the muffler shop tell you they cannot run 3" pipe. Guys have run 4" pipes on 68-69-70 Chevelles.
Running a free flowing exhaust sytem will make your engine get into boost faster as does a free flowing intake filter, the larger/longer , the better.
You need as free flowing exhaust sytem as possible, meaning, you dont go to a muffler shop & have them crush bend the tubing (that restricts flow), you run mandrel bent tubing & a free flowing muffler (straight through design) You want minimum bends & never have 90 degree bends if possible.
Companies, make universal up & over the rear end dual exhaust systems. The one up & over the rear end for my Syclone was made for a 70 Chevelle & just bought for one side.
Your wastegate plumbing of the pipe to the wastegate looks like a good angle.
I would not recommend you race your car @ the track so soon. You need to do a lot of tuning first before you should even consider flooring it.
