Jerry,
Drill the 6 additional water passage holes in the 235 head. Otherwise, water will not circulate between the block and head as was apparently needed to cool the 3 pairs of siamesed cylinders (GM must have thought that these extra water passages are necessary).
I agree, Tim... I work in an Engineering field and can also appreciate any features of design.
As I understand it, the 261 springs are slightly longer too... probably to get a greater valve seal. I haven't disassembled the head yet, but I'll check the spring length while I'm at it.
Jerry
