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I have 2 53 inch drive shafts for the the '73 GTO. One is a 2 inch and the other a 3 inch

The th400 yoke is already on the 2 inch shaft from a Monte Carlo. Is it worth my while to switch it over to the 3 inch shaft?

Running a 1970 Pontiac 350 4 barrel, mild cam, headers, probably around 290-300hp...
 

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I would say no. There should be very little streetability improvement with a 3 inch shaft. It might even hurt with the increase in rotating mass. If the 2" works, use it.
 

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have yet to run across a 2" diameter stock GM driveshaft of that era, sounds mighty weak. A stock '73 GTO with turbo 400 or Muncie 4spd is going to have a 1piece driveshaft (none of the rubber isolated 2 piece nonsense) & the diameter of that "73" driveshaft is going to be 3". IF, after several inches from the ends, the driveshaft steps up in diameter a few inches, that center area, will be near 3 3/4" in diameter. Thinking back, dont believe the '73 style driveshafts have the step up design. Its been years, but I used to pull straight '73-75 2door LeMans/GP/GTO turbo 400 length driveshafts for next to nothing, & have them cut down to make correct length turbo 400 driveshafts for late 2nd Gen Birds. FWIW, the original '73 GTO/GP driveshaft will have 3R ujoints, '73 & 74 Chevy A-bodys will have the smaller spicer 1310 ujoints.

Quickly looking at oem GM driveshafts, the smallest diameter stock driveshaft in my driveshaft barrels is one from a '78-87 ElCamino. I kept it in order to note the stock length. Pulled it out of one of the first ElCo's I built an 8.5 G-body rear for, & then went with a 3" diameter driveshaft that was 7/8" shorter. no way I'd install that flimsy thing behind a Pontiac building any kind or torque.
 

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Can I change my answer? LOL

I was thinking about this overnight and thought about the 2" thing. 2" seems pretty narrow. But the thickness of the tube wall is key.
 
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