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79 Chevy tranny swap

Discussion in 'The Garage' started by 79Stepside, Jun 29, 2005.

  1. 79Stepside

    79Stepside Newbie

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    Johnstown, PA
    I have a 1979 Chevy Stepside with a TH350/NP203 tranny/t-case. I am currently building a 383ci motor for it. It will be in the 300-400hp range when I am done. While I'm at it, I'm thinking about swapping a 700r4/NP208 combo in. I found a just rebuild 700r4 tranny with the NP208 for $150. And I get the driveshafts, linkages and whatever else I need with it. I'm thinking about doing this because I want an overdrive since this will be my tow unit and I want a part-time t-case. I'm told that I'll have to beef-up the 700r4 tranny....what will I need to beef up? Would it be worth it even? Should I stick with the TH350 tranny. It has 3.08 axle gears and a 4in lift with 33in rubbers. Since the 700R4 is 2inches longer, I will need new drivshaft lengths.....I'm thinking that I will be able to use my current shafts because it looks like they slide in and out and should adjust for the extry tranny length?
     
  2. 79cheyenneblazer

    79cheyenneblazer Registered Member

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    I'd stick with what you got, if you need a stronger tranny put in a TH400, should bolt right up with the 203, or even go with a 400/205 if you want.
     
  3. LongIslander26

    LongIslander26 1/2 ton status

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    700-r4

    With 3.08's and 33's, that 700 would have a life expectancy of about 10 minutes the minute you got silly with that 383's gas pedal. You also would need to have the driveshafts done because one needs to be made longer, and one needs to made shorter. It's not the horsepower the engine makes you need to worry about, but torque, especially from a dead stop, and those 33's are going to make your 3.08's seem like 2.08's because of the size difference in the tires, and if your running a 10 bolt, your just asking for trouble eventually. Getting a nice new hi-po motor is never a bad thing, but it is if the rest of your drivetrain can't handle it, your just asking for headaches. Plus, if you want to run overdrive with those big rear tires, you're going to need to change out your axle gearing to at least 3.73's to get anything even close to decent performance, otherwise your gearing is just going to bog down the motor, and all that stress on the tranny is just going to make it go "boom"!
     
  4. seschev3

    seschev3 1/2 ton status

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    If $ not a problem , do the swap, but do it right. With 33's, I would go with 4.10's for gears. If you are towing, your tranny needs a serious build. I tow a 27' trailer with 4.88's and 38.5" tires and had a tranny built ($2000) into it. It blew up. Since I have done all of the mods for the 700r4 in my 75', I have once again bought another 700r4, this time from Bowtie overdrives ( level 3 $1700 ) that I am going to try. 1st tranny was a local build that I was told would be bulletproof, so now I have my $ ridding on Bowtie to see if it can handle what I throw at it. Blazerfest is going to be a good test in a couple weeks. I have found that if you want to go with the swap, just buy the tranny built by a well known company, but it is not a cheap swap. You need new crossmember, carb linkage, driveshafts, cooler, heavy duty torque converter, and other misc. items for the swap. If you want a tough dependable tranny for cheap, go with a th400 built and save the headache. Just my story, hope it helps with your decesion. Good luck and if my new tranny blows up you will read about it here in the garage.
     

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