Hi. Newbie here. Got a 67 with a 350/350TH 3sp auto, 3.73, b&m Hole SHot conv around 2500 stall,. Great off line but miserabley high rpm @55 mph on hiway. No Tach but must be in 3500 to 4000. Can watch the fuel guage move. I didnt build it, but can tell I need to either lose the converter, change the gear, both, or find a built 700r-4. Would love OD. I bought a race car, but want more of a cruiser/best of both worlds situation. Neigbor suggested the built/tunable 700r-4 fix. Im not a racer but this thing is killer off the line and thats about it. Not fun to cruise around in, to many rpms for hiway.

Any major mods required to throw a beefed up 700r4 in it? ANy advice is greatly appreciated.

You either have a bad converter and/or a very small rear tire (like 24 inches tall) if you are turning that many RPM's.
I have a T-350 with a 3.90 rear gear and a 2800 stall converter and 27 inch tall tires. At 60 MPH I am only spinning 2950-3000 RPM's. I drive this car daily to and from work down I20 in Texas,

I would recommend the 700R4 route because you also get a lower first gear and an overdrive. Place here in Weatherford Texas (Phoenix Transmission) builds a stout 700 and ships all over the country.

no major modifications are required to install a 700r4, even less is required to install a 200 4r.
Well, basically the difference is that you might be able to use your stock drive shaft and crossmember.

Either will need either new detents on the shifter or a new shifter to accommodate the extra gear. A tv cable, bowtieoverdrives.com sells a great kit that I used when I installed my 700. And a new converter to work with the lock up trans.
I am not sure which trans but the 200 4r is the same length as some of the stock transmissions available and can reuse the stock driveshaft and crossmember, but a 700r4 will need new for both.

I have 4.10's and a 24.4" overall tire and I spin 3098 @55mph. Either you are being thrown off by the sound or like Tony said, something is wrong with converter or tranny. First thing you need to do is get a tach. No car is a car without a tach. Then, get your overall tire diameter and go to this website:
http://www.xse.com/leres/ss/calculator.html

You can enter your info and get your real running RPM.

Is it possible you may have something steaper than a 3.73? I've had a similar combo years ago (TH400 & 3.73 w/275 15s) and have to say it was definitely no hiway friendly cruiser by any stretch. Felt like I was needing to shift into another gear that simply wasn't there and I couldn't stand putting myself and the engine through that. I kept the 3.73s (had to cause I have a series-4 12 bolt that won't accept lower ratio # than 3.73) and installed a 700R and then was great - ahhh free again:hurray:. I just don't see how people are content on the hiway w/3spd. autos and 3.73s or higher. It just doesn't work work well in reality. Now I've got the 5spd. and like it even better - spins like 2,200rpm at 75mph and smooth as silk.

I think you'd be be very satisfied with either the 200R or 700R and 3.73s - assuming that's correct.