The only difference that I can find (so far) in engine specs between 1957 and 1958 truck 235 engines are in the valve springs:
235 & 261
1956/1957 - 74 lbs at 1 27/32" (2.156 free length) - Rotocoil valve rotators only in 261
1958 - 194-204 lbs at 1 29/64" (open length) - Rotocoil valve rotators in 235 & 261
It should also be noted that the 261's compression changed from 7.8 to 8.0 between 1956/1957 and 1958.
Are the rocker arms different? I'll look for more differences.
