I an replacing my rearend with another, and it has 3.73's in it. Stock rear with 2.73s or whatever was in it. Its a dog off the line, but gets up to speed and cruises nice.

Mild 360 with Offy intake, 650 Holley, TTI headers, mild cam, stock 904 with the 318 TQ... no tach, but it doesn't sound like it's running that hard around 75-80... will I still be able to roadtrip comfortably without doing any harm? How much of a difference should it really make?

On the plus side, I am looking forward to some two-wheeled burnouts for a change.

Use this and you will have your answer. http://vexer.com/automotive-tools/speed-rpm-calculator

I run 3.73's behind my 383, with 28" tall tires and a 2200 RPM torque converter. I recently took a 400 mile round trip. Cruised at 3,000 RPM's on the interstate which is about 65 mph. No problems. Been running these gears for several years. I like 'em.

Hmmm.... but it will get out of it's own way off the line, though, right?

I wonder if I can get an Overdrive trans? LOL... I like to drive fast.

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but it will get out of it's own way off the line

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You should notice quite a difference from a standstill, much more responsive.

Your cam and converter stall speed and rear tire height ALL play into this equation. You should notice some performance improvement now, but ALL the above components MUST match the rear gear ratio before you can run with the big dogs.

Interesting...

My cam specs:

936P
Intake, exhaust lift: .443/.465
280/290-advertised duration
Intake and exhaust duration @ 50: 214/224
61º overlap
107º lobe center

Tires are 15x10 275/60/15, and the torque converter is the stock one from the 318 on a B&M flex plate....

How does all that fare with the new gear, and what would you change? Is there anything seriously wrong? I know it's a big car, and I take long trips... I'm not looking to run in the 10s. just have some reliable fun. It's a one-legged dog right now until about 70mph...

to be fast at the track, you want your converter stall speed rpm to be close to your maximum horsepower rpm. When you leave the green light, you want your stall speed right at maximum horsepower (so the engine doesn't need to spool up to max hp, you are already there). You shouldn't be traction limited with a early 70's B body, they were made for extreme wide tires. Your stock converter is probably your limiting factor right now, the converter is stalling well before your motor is making maximum power. On a budget, step up to a mopar 175k converter. If you can get an extra $600, step up to a good performance converter. You'll need to add a big trans cooler (if you haven't already) to keep things cool, but she should then jump out of the hole, WAY before 70 mph.

It will feel like you hit it with a 100 shot.
You may have to retune the carb somewhat for optimal performance.
You will be spinning a lot more rpm on the highway and will lose a lot of topend potential.

Would a 3.23 make any difference? Is that a nice average type of gear for a car my size? Like I said, I don't want to go drag racing, and I make frequent road trips... but the 2.70s are a dog until it gets rolling.

3.23 & a good all around gear.. Cruises well & not terrible around town.. With a smallblock I'd probably consider going 3.55...

3.73 ratio will be great for street performance but you'll still be able to cruise the highways.

Good choice.

Play with the calculator stumpy linked you to. Really building an overdrive transmission is the only way to get the best of both. With a combo like yours and a 727 or 904, everything else is a compromise.

I drove to the shop in 2nd, like suggested, and while it was very peppy I felt like I was over-running the engine for the way I drive. I make frequent road trips on roads where 75-80 is normal and don't think I would like the 3.73s... those were in an axle I was going to swap out my 8 1/4 with... but then the guy at the shop had a buddy with a big block/auto 73 RR parts car that is missing the engine and trans, but has the 8 3/4 rear and front and rear sway bars, hood, passenger side mirror, and a ton of other stuff I can use, so I'm buying it. I was told that the rearend has 3.23s but they don't know if it's a sure grip or not. That would be cool if it was.

Go with the 3.23!
Set up the engine and tranny accordingly.
You'll be one happy cruiser.