I swapped a 14bsf into my 6.5 blazer with no converstion joint, others have said that it is necessary, so I am forced to assume there is a difference in the 6.5l driveshaft (it does have 4l80). So unless you're diesel, I would assume you need one based on what i've read.
Other than that, the only major considering is that the 14bsf is longer from axle center-line to yoke, so if you are not lifted, you will very likely destroy your t-case output seal due to rust rubbing on it.
As for the 8 lug swap front end... you already figured out, NOT worth it. This does stop you from using a 14bff in the rear unless you're willing to mismatch wheels, but the semi-floater is plenty strong for anything but high power wheeling.