Full flow filters really don't do as good a job of cleaning up the oil as a bypass filter. It's possible to run both types, with a VERY tight-weave filter element in the bypass canister, but a plain old Fram or AC bypass filter will do the job quite well by itself without going to the trouble and expense of adding a full flow.
It would be a very good idea to use the water pump adapter plate, as using the low-mounted pump in your truck will create fan position issues and poor cooling. A 216 or 235 harmonic balancer is a direct swap. Use whichever balancer, water pump and charging system that will have the correct belt width for all the components. If you decide to keep the 12V flywheel, you'll also need to go with a 12V starter. It will have a shaft diameter of 1/2" instead of the 5/8" shaft of the 6 volt unit. Of course, that will also require converting the whole truck to 12V.
The 850 casting number cylinder head is correct for a 261. If you choose to use an 848 head, read the tech tip on how to drill the "steam" holes for correct coolant flow. It will increase the compression and result in a little more power and torque.
Jerry
