On 2 of my cars, I run 5.25X21 tyres on the back and a stock 3,78 rear end. That gives me an effective ratio about the same as running a 3.54 diff (to within 1%). By the time I engage the 26% Mitchell O/D, the effective ratio is 2.81 - way higher than anything spoken about so far. There have been times when I have forgotten to take the car out of O/D when I stop and try to move off in high 1st. It doesn't like it but once moving, I love high 2nd and high 3rd gears. Even towing a trailer that weighs as much as the car, 50 mph on the flat is easy.
The motor has a 5.5 head on it but I'm sure it is higher now that it's been planed a few times. It cranks 105 - 110 psi compression. It has a mildly modified cam and a down draught Stromberg carburettor that has a throat the same as the first barrel of a Webber (26mm) I estimate it puts out about 70 hp.
That combination has been all around this continent (same size as your lower 48) a couple of times and more and loves the flat outback country. Around these parts, I don't tow very much and that setup is great for solo car touring. I wouldn't change what I have for versatility.
In short, with the right engine, 3.25 should be good ratio but with a less modified one, I wouldn't go higher than 3.54 and be prepared to travel slower.
It is worth noting that the engine will be working harder with a high ratio and cooling could then become an issue. Ask the man who knows!
