The 2.8L V6 used a different bellhousing than the 302cid (5.0L) V8. The only engines that will swap while keeping your current bellhousing/
tranny are the Cologne V6 engines (2.8L, 2.9L 3.0L RWD, 4.0L, 4.0L SOHC) all of those will swap in, but with varying degrees of difficulty, ranging from simply finding the parts, to wiring the electronics with the 3.0 and the two 4.0s.
If you really want to go with a V8, you'll need to find out which of the RAD-4 transmissions you have. Most Mustang IIs got the American RAD-4 with the 10-spline input shaft, but some (mostly 2.8L V6 Mustang IIs, because the engines came from Germany) got the German/European RAD-4 with it's 23-spline input shaft.
If you've got the 10-spline, simply find a 302/4spd bellhousing and flywheel (these are Mustang II specific unless you want to beat the firewall with a sledgehammer to clear a standard Ford bellhousing, and they're also nearly impossible and very expensive to get ahold of because Ford doesn't make either one anymore, and neither does anyone else) a set of Mustang II 302 headers or exhaust manifolds (you won't save any money going stock here, as your average junkyard wants upwards of $400 for a set of Mustang II V8 manifolds, and Headman sells headers for as little as $185... but the Headmans have a bad reputation for leaking, leaving the $400 Hooker headers as the best bet.) Lastly, you'll need Mustang II V8 motor mounts and frame mounts... again nobody makes these... and even worse, no junkyard I've called yet has any... I bought a set on Ebay (used, and well-worn) and it cost me nearly $200, and my brethren at a sister Mustang site have mostly reported paying even more for some in similar condition... there is a Canadian company that will replace the rubbers in a broken set of the engine mounts, but they're done on an exchange basis only.
You'll also want to go with a 1981 or older 302 because the crankshafts on the 1982 and later engines are balanced differently and won't work properly with the Mustang II flywheel.
Not trying to discourage you, but these cars are getting harder to restore everyday. I've had mine five years, and for now it's got a new 2.3 because I'm having so much trouble gathering the V8 parts (I've got a 289 with a set of E7TE Mustang 5.0 heads, and some other parts gathered, along with my motor mounts and frame mounts, but it's gonna be finding the bellhousing and flywheel that determines when my '74 gets it all put in, and it's a in-demand bellhousing and flywheel, finding it's way into Rangers, Pintos, Bobcats, Capris, and other small cars with 289/302 swaps.
convert v6 to v8 
, the question is can I save transmission box ? Will it fit ? If not what kind of work/modifications do you think I would have to do ? 
