Correct me if I'm wrong, but the original 1955-57 Thunderbird driveshaft diameter is two inches. For those of you who are now making in the neighborhood of 300 hp, have you gone with a larger diameter driveshaft? There seems to be plenty of tunnel/exhaust clearance for a larger diameter driveshaft. I don't expect rpm to exceed 6,000, if that, and seldom seen. I'm replacing the Fordomatic with a C4, using a Flat-O Products adapter kit. Gene says the driveshaft needs to be lengthened 3 inches. If you've been down this road, what do you recommend?

400+horsepower. Stock driveshaft with new U joints when I built I a few years back. No issues yet. I’ll know more when I get slicks on it. 

Thanks. PST's 2-inch DOM driveshaft is rated to 500 hp and uses 1310 u-joints. I might go that route. 

I just last year installed my 345Y and T-5 with new driveshaft made from original style tubing and new universal joints.  My engine made 402hp with headers and 367 with manifolds.  It runs just fine and is very quiet at speed.  I was dubious at first, but so far it has worked just great, and I have pushed it a few times just to see how the new power feels.  It puts a grin on my face every time.  Joe-JDC

JDC

I've bent my share of 2" driveshafts in the past with slicks in a stick bird with 200 and up hp. The automatic should be easier on the driveshaft though. If you are going to pay to change the length anyway I would recommend going with a larger diameter.

  Vic Correnti

No slicks or track time for me, especially with highway gears.

I ran the same original driveshaft in the late 60’s with a 300 horsepower dual quad 312. I was launching it at 5,500 rpm’s with cheater slicks at the Alton Illinois drag strip. It’ never bent or broke. Was shifting at 6,500 rpm. Had a set of home made Traction Masters. Running original Traction Masters now. 

Blacktie,
I have 2 - 3" diameter driveshafts if your interested 1 is 28-3/8" long and the other is 31" long to u-joint centerlines .

  Vic Correnti