Toby's transmissions in Florida builds a great trans a 700r4 w/torque converter stage 3 for my 350 , not stock, around 400 HP , the cost was around 2200. He guarantees his builds 2 years. He is on eBay & has his own website
I have a 455 / th400 combo works pretty well. I get like 6 to 7 mpg but I also have a Ford 9in with 3.55 gears. The low miles per gallon helps me out i get offers for people to buy my car all the time. I tell them sure you it get 6 or 7 mpg and the are like um for get it I know full well they won't buy it. I'd never really well it anyway. Not sure if any one shave brought it up. But if your that worried about mpg look in to a Gear Vendors over drive. It's like bolting a second trans to the back of the th400.
Since it's 99% street you don't need 3.42 or 3.73 gears. They are nice when putting around town, but you don't need them with all that torque. Then you pay the price when out on the highway trying to cruise at 65 or 70. The 200-4r would be nice, but throwing $3400+ at one to get it to hold up isn't. Use the turbo 400 and swap to 3.08 gears. That's what i'd do.
Stock GN 200-4R with the GN converter. I would go to 3.42 as the GN govenor is matched to that rear end gear ratio. Mone SS and 442 use a govenor to match 3.73.
Just to touch on what Oldsmobile offered in the old days, an Olds 98 that weighed 4800 pounds and had a trunk large enough to park a G-body in, would have 3.08 or 2.73 gears and effortlessly drag the car down the highway and get decent mileage. They also used the Toro 455/425 transaxle in many a GM motorhome without overdrive. Later 454/TH400 RV's did come with a Gear Vendor unit as production.
If you already have it put it in, but, that deep gear, you don't need it, you won't like it. 3.73 in Monte SS and 442 was to feel like the 305 or 307 had guts like a big motor. But, it's your car. I had an 85 Cutlass with a stock 73 Toro 455/TH350 and 3.73, it gets no mileage, and will destroy a set of tires in a month. When the 455 let go (Teflon timing gears clogged up the oil pump screen) I swapped in a 403. My dad still has it and complains about the gear and no overdrive all the time.
I am really not sure as I have never had the car dyno'd, it was just a conservative guess. But I've been in my friends GN and that thing pulls as much as mine, they sure feel like a V8 with that small turbo on there.
If you can't get ahold of a gn 2004r the next best one is out of a late 88 or 89 caprice since it had all the upgrades like the gn version but a lot cheaper.
Most of us have never heard of this Caprice version, that is why no doubt. I will be surprised if it shifts above 4000 rpm full throttle, unlike the GN's 5000 ish rpm. Mine with the Transgo green TV limit spring plus a very lightened small weight and the spring pinned in still only shifts at 4500 rpm, I was hoping for 5000 rpm. Also does it have a better servo as well? Those along with stiffer springs, if you have the Performance versions, GN, Monte SS, 442, Transgo tells you to leave the stock springs in place as they do the same as the firmer, higher shift point version in the shift kit.
Mileage depends more on the rear gear ratio than the transmission. With my TH400 and 3.42's im buzzing about 2400 rpm at cruise. Not bad for what it is considering no one builds big blocks and worries about fuel mileage. An overdrive would be nice but it would take forever for it to pay it's self off in fuel savings.
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