If you read Pre Dave's blog in the paragraph above the bit about spinning to quick it reads the following quote...
1941-52 Harmonic balancer and water pump for ⅝" belt.
1953-54 Harmonic balancer* and water pump for ⅜" belt.
* For ⅜" belt an acceptable substitution is to use a 1955-62 truck balancer. This balancer is designed for a ½” belt but may be used with a ⅜" belt. This will allow the belt to run deeper in the groove and in affect will reduce the diameter to very near the correct size.
So by running the truck balancer the difference of 1/8 isn't in the diameter but the width of the pully groove allowing the 3/8 belt to drop deeper into it and in turn reducing the overall diameter that the belt is turning around., as also mentioned by Pre Dave in that blue highlighted quote.
I used the balancer from my 216 for this application. I run a more modern flex fan, a 15" PermaCool to be exact. 6 blades, and it was very close to kissing the larger diameter 235 pulley. That's why I pulled the one off my 216. Runs great.
Good luck, take your time, and don't drill into the block too deep...Phil
