The 61 would have had a hydraulic clutch. The clutch fork was operated by a slave cylinder mounted to the passenger side of the bell housing. There was a master cylinder on the firewall with two ports, one tube went to brakes and the other tube went to the clutch slave cylinder. You probably have a bell housing now that has the clutch fork on the drivers side. GMC was hydraulic from 60-63, then mechanical.

You now are in the Chevy world. It is probably a mechanical clutch on drivers side, don't know, no pics. If it is, you will need to get a donor bell crank and rods from a 64-66 GMC or I think all light Chevy trucks had mechanical 60-66. You will have to study the situation. I believe your options are:
1. New drivers side aftermarket hydraulic clutch kit.
2. Donor clutch pedal/brake pedal, linkage, bell crank and fork.
3. Adapt GMC pedals to whatever you have or want.

You need to get a GMC manual reprint or used original From Ebay. 60-61 same manual. (N series) X-6003 Click.

I can't answer the clutch 420 465 questions without research. There is a bell housing/tranny pilot diameter issue. Go to Novak web site and read all about it.

This is a bell crank , called Z bar by the cool crowd. Get hip to ebay for donor parts. Junk yards are gone or stripped. No mask required on Ebay.

Another

There is an assembly manual for 60-66 Chevy/GMC. Don't know if you need that or not. Ebay.

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