[LH Lead-Foot]

What are you putting in for engine & tranny?

I am sure if you asked a drivetrain expert, they would tell you to go one piece. You don't have to deal with support bearing, plus 3 other u-joints. I'm sure there a few locations that will do this for you...for a price.

I am used a 1 piece drive shaft from an 03 Tahoe. It is steel and slightly larger diameter compared to the OE 67. It has the same 27 spline slip yoke that fits my 1987 700R4 behind a 5.3L "LS" motor. The u-joints are larger but few 50 year old trucks would not need newer u-joints.
A little research let me find the Tahoe is 3150, while the OEM 1967 is a 3110
(I think the numbers are right, but double check them by measurements and online charts)

Always mark the location of the yoke's centerline before cutting off to reference proper location.
I cut the yoke flange (Companion Flange) off at the weld, cut shaft to length, cleaned, ground an angle on cut edges and welded. I had help with "Tee Notch" jig and someone to turn the shaft slowly as I welded it with my 1984 MIG.
(This MIG is huge with it own cart, tank ledge and handle. 155 amps and hold the 14" spool) New ones are real small. Maybe that's why it's still working.

Afterwards, I install a conversion u-joint that is 3150 on one side & 3110 on the the other. Oddly enough, it's OEM on the 1988 - 1999 Camero and Firebird and commonly available. I got mine from RockAuto for $7 from spicer.

As far as balancing, you can use the GM factory service manual trick, using two worm-drive hose clamps mounted at the back, then rotate together to find the sweet spot. Just make sure they are one clamp, even if you have to buy two that a large enough to fit.

If you safely support the vehicles rear axle with stands, blocks on the front, these clamps will quickly find balance as it will not take much for them to vibrate the ale & frame. Just leave the clamps loose enough to move, so you can do this in 15 minutes. Tack weld two washers for 5/16" bolts where the clamps land.

It's a two page process in the GM service manual. I got lucky & didn't need anything added to balance.