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We have a '66 Lemans with a 326 automatic. The transmission is a Jetaway 300 (I assume a th300?) which is tight and leak free. I recently acquired a 1971 Pontiac 455 engine with a th400, also good shape. I was wondering if the 455 will bolt directly onto that Jetaway (will the torque converter bolt up to the flywheel, housing bolts align, etc...),so as to avoid modifying the transmission mount? Will the hp in the stock 455 be too much for the Jetaway? Also, can I use the exhaust manifolds from the 326 on the 455? (unbolt the 326 exhaust and leave it in the car and slip the 455 right in? (to avoid having to change flanges). Thanks very much. gjones
 

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The 2 speed auto in a Lemans is a SuperTurbine 300. A Jetaway is an Olds unit. I would bolt the 455 and the TH400 in. You'll need a driveshaft fabbed and to relocate the crossmember. A 2speed ST300 has a 1.87 first gear and a 1.1 final. A TH 400 has a 2.48 first gear.....faaaaar better acceleration, and stronger by far, too. Torque converters don't swap, either, as far as I know. The one best thing you can do for your car to improve performance is to swap out the 2 speed. In fact, I'd almost be that a 326 engine with a TH400 or TH350 would blow the doors off of a 455 engine with the 2 speed....at least for the first 1/8 mile or so!! To answer your original question, though, yes, all bolt patterns on the engines are the same, so the 2 spd will bolt up.
 

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Discussion Starter #3
66 Lemans questions

Thanks Geeteeohguy for a quick response to my first posting on the forum. I absolutely understand that a th400 is far superior to any 2 speed transmission, and I've had several th400's over the years in Cadillacs, Chevy's, etc..., but remembered the old gearheads actually liked the powerglides for the torque? (maybe I was just dreaming). Having resurrected this Lemans slowly for the past straight year, I guess I am getting lazy, and was hoping to just slap the 455 right in, bolt the existing exhaust manifolds on to avoid flange swaps, and attach the torque converter to flywheel bolts to avoid the trans. mount and driveshaft thing. (already been there,done that-"we do it nice,'cause we do it twice"). What about the driveshaft yoke? Is there a common swap for this part? Also, this 455 was sold to us as a 1975 motor, however, the #'s on the distributor pad say do51, with a large 71 next to the dist. hole.(is this 4/5/71 ?). Near that on the right rear is # 485428. On the front right of the block are #355304 yc. I've tried to look these up, but have had limited success. Heads are 4x with # g203 (7/20/73 ?). Cylinders are 30 over with flat top pistons, with stock cam. What kind of hp roughly would this put out? Thanks very much. More questions to come...
 
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