That seems pretty low. He will be lucky to run 14's LOL.
You are going to smoke that weak 408!
If your SPA put the turbo close to the stearing shaft, you can always upgrade & use a bolt in rack & pinion stearing unit, should give you more room I would think?
But,,,, do your homework.
Dont know where anyone said the stock 2.5" manifold flows bad for a turbo set-up.
Mileage,, are you running stock timing? Is your choke partially closed? Are you flooring it everywhere you go?
How is the compression? Tires inflated properly? Front end is perfect/alignment? Carb worn out, needle worn, etc.
Best my moms stock 73 Nova got on the freeway was 16 MPG. Moving the timing around made a huge difference in power & mileage.
Run an MSD box & an adjustable timing control & play with your timing.
