Does the propshaft for a BEC require a sliding joint to allow for the slight movements of the diff and engine? If so, how have people done this or am I as well spending my time earning some money to hand over to Mr. Keenan for one of his?

The MK joint only slides to adjust the length intially, you lock it up once adjusted to the correct length

if you have a live axle then yes the prop needs a slide, irs you can get away with it.