Tire size has alot to do with it. IIRC the Buick GNs had a 24" (ish) tire with their 3.42s.
To know you would have to have the expected vehicle weight (best SWAG), tire size, overdrive ratio (.67 for the 2004R IIRC), and some idea how much drag you have (if you plan on driving on the highway). The drag only has to be a general idea, I have a Model A, it drives like a parachute at speed, a Golden Hawk (just to pick an example) would be much more aerodaynamic (so, what was it you are puting this in, did I miss that?).
Lugging is lugging, a turbo will not help that. For purpose of clarity, for this discussion, I consider lugging anything that would require you to run with the power enrichment on during cruise (carb or EFI). A turbo (or any forced induction) will only require further enrichment.
That would be cruise on more or less level ground, no excessive headwind, not 'drafting' a truck, just regular cruise driving.
I'm not saying you shouldn't use a turbo, I'm just saying you should be careful to match the gearing to the aplication.
I asked alot of people I trusted what gears I should run, and got more or less the same answer (3.55 as the highest number). Pops about had a coronary when I told him I had 3.70's, telling me I shouldn't expect any economy at all (because I would be winding it too high). He was right about the economy (in 5th) but for the opposite reason.
That problematic advice seems to be standard, it's a throwback to 1:1 final in the trans and 23-25" tires on the drive end (I've got 27.8" on the A, call them 28). It goes right along with the whole 'if some is good, more is better' philosophy. As usual, a properly matched setup performs better.
FWIW I'm trying to scrounge a set of 4.11s or 4.30s (9" ford, 31 spline) to try as with that tall a tire, not only is it too tall geared at cruise, it's too tall in 1st, I'm favoring the 4.30's (because of the extra drag, a few more RPMs with the throttle more closed will tend to give better economy). Of course ther's no real way to tell for sure, untill you screw everything together and drive it. Replicating a similar build is close, but unless the build is identical there's no gurantee.
