What year is the 261? Just for info (you may already know this...) but the early 261's don't need the adapter plate and all the peripherals (water pump, thermostat housing, etc) switch over from the 216 without any mods needed.
Just sayin 'cause I just put a '54 261 in my truck ('49) and didn't need to do any mods to it at all. The water pump and the thermo housing from the 216 fit perfectly and in the lower position.
Regards,
John
