I am about complete with an install of a 55-57 261 in my '53 3100, and here are a few things I have encountered. Please correct me if I word something incorrectly:
1. Use the Pre-68 Dave adaptor plate and 216 water pump to allow for proper fan placement. The machine shop can drill the needed holes for adaptor plate installment. But make sure the harmonic balancer accepts a 3/8" belt.
2. The 261 will not have the front motor mount holes drilled behind the thermostat housing. These will need to be drilled prior to installment so that the 216 front motor mount can be used.
3. A Delco 10-SI alternator can be used if the truck is converted fully to 12 volts.
4. The 216 bellhousing will need to be used, or else the 261 bellhousing will need to be modified for the rear motor mounts.
5. Thermostat housing from the 216 can be used with 180 degree thermostat.
6. The 216 fuel pump can be used.
This is what came to mind initially...I'll think of more later.
