FWIW Rock Auto's flywheel listing by application includes a flywheel for 235/261 L6. I ordered one and got the correct part number in the correct box but it's the SBC flywheel and can't be readily adapted to an I6 235/261. I sent it back with an explanation of the error in their application listing. That was 2 years ago. Chevrolet made a 262 V8 listed as 4.3 as is the 261 L6. That's probably where their parts people got confused.

I was looking to replace my 9-bolt 10-3/4" clutch with the still available 6-bolt 11 inch clutch. It was a no go on my search for a flywheel so I had the 9-bolt flywheel resurfaced and all is well. The machinist who did my flywheel suggested that When the time came to replace the clutch that I just have the 9-bolt flywheel re-drilled for 6 bolts.

Also note from the parts books, that the 54 flywheel is unique to that year and the bell housing changed in 54 also. So you can't use a 54 bellhousing in earlier trucks because the frame crossmember is different. The 54 flywheel has the 6V tooth count on the ring gear and 6 clutch bolts so you could use it in earlier trucks with a 6-bolt, 11" clutch. 55 and on flywheel has 168 teeth for 12V starters and 6 clutch bolt holes so you'd have to change starters along with the clutch and flywheel to use that flywheel in 54 and earlier trucks.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.