My 84 Bronco 2 was a wreck when I bought it, and it ran like crap...one day it was so bad it kept acting like it was going to die. I pulled over because I kept hearing a clicking noise on the passenger side. I ended up ripping out the relay that was by the computer. It ran a lot better afterwards. The duraspark conversion will help a lot.
You said 2.9L V6 which unless someone swapped in that engine your B2 should have a 2.8L carbureted V6 in it. If the 2.8L engine's carburetor isn't adjusted properly it will act like it has no power, it may run like crap, hard to start, hard to keep running, etc.
For swapping engines, unless you can do it yourself, have the tools to do it, and the room I wouldn't suggest it. The 2.8L V6 will do just fine contrary to what many will say or recommend on this forum LOL.
Throwing in a 302 or the 4.0L V6 will just get you less fuel economy, and more headaches...when you start swapping engines that don't belong you end up with a vehicle that may or may not be so reliable, or may give you issues randomly. Sure the swap can be done, but unless you have a free or very cheap donor vehicle its not worth swapping. I had 2 RBV's with the 2.8L V6, and the Ranger towed a 14' travel trailer all over the country for many years...my grandfather had it at the time and even with the little V6 he was always leaving their friend way behind who had a 1/2 ton Dodge pickup with the 318 V8. That little V6 will run forever if you keep it tuned up and serviced properly. My Bronco 2 that I had for about 2 years the body was in just horrible condition, far more work to repair than I had money, then came the starter issue which was caused by the previous owner screwing around with stuff on it put in the wrong flywheel so the starter never lined up properly...wasn't about to tear into that mess for a 3rd time LOL.