If your L48 already has 2.5 true duals hooked to OE vette manifolds you're not going to see much gain. Maybe 10 hp at some point on the power curve, but probably not at the top.
What it will do is make the L48 holler & sound bad*ss. From the driver's seat, that's what most of the change will be. Sound, not power
As our cars already have decent manifolds, I wouldn't install cheap hedders because they rust and look so bad, so fast. Hedman elites are a pretty good value for coated hedders. Coating still doesn't end rust.
If you're thinking about some fancy hedder paint, (por-20)those only work when the manufacturing oils have been baked off first. Major PITA to do so.
1 5/8 is preferred for the majority of street small blocks. If you're making BBC power with a SBC, then it's time to think about the larger primary tubes.
good luck with the project