JMO, you'll lose more than 50 HP engine size at 6000rpm,but not just because of the 1 5/8" pipes. the 383 might peak below that rpm.
from 1500-6000rpm, your engine will need 665cfm or more, and that single plane manifold will not perform its optimum as an rpm intake(or similar), especially with a teeny 600 carb on a street car.
my choice: 700cfm carb/edelbrock rpm intake(or equivalent)/1.75" headers/2.5" FULL exhaust system.
on DDD2000, the single-plane intake will show an advantage over the dual-plane, with any carb size from 390-850cfm. in reality, it would be a disappointing mismatch for a street-strip.