#3 What size (length) are the thermostat housing mounting bolts? I have a couple of bolts that thread in about 3 full turns. Is that enough? If not, should I run a tap/die through the holes?
Those bolts are long enough. I would NOT stud the thermostat housing.
I do not know the exact length of the OE bolts. But 3 full turns provides enough clamping force to do the job.
Running a tap, not a die, is what you would do to clean the threads up in the bolt hole. It's not a bad idea and it should be done if there is resistance in installing the bolt by hand. It's bee a while since I have owned that intake. I do not remember if the bolt holes are blind or go into the water passage under the thermostat. If it does go into the violent passage, wrP the bolt(s) with Teflon tape.
That's a good intake you have there. I ran that with the same carb your going to put on there as well.
Oh, I forgot to mention, that water pipe at the front of the intake should be a simple straight up pipe for the heater hose.