the bellhousings changed in 68 with the intro of the 302 - so all 302's are the new bolt pattern - - it is a larger bellhosing so yo will also need a larger flywheel (probably already on the 302)

289 and 302 motor mounts are the same - i put a 302 in a 63 galaxie and found out all this stuff the hard way! Never tried to replace a 6 with an 8, so i don't know, but would suspect that the six has different mounts.

you could try searching for van conversions - the fist gen econolines came wit a 6 only and putting an 8 in them was popular a while back - if you can find a web site about it you will see what all you need.