1. Only the 235 or a 261 will bolt up to your current belhousing and motor mounts.
2. If you use any other engine than a 235/261, then the bellhousing, starter, flywheel, pressure plate, clutch fork, will all need to change. tranny wouldn't have to change, unless you want to.
3. Don't know anything about it. Call them directly and find out.
Try finding a 63-66 truck in the salvage yard, etc.. that has a six in it for comparison.
