ive heard that the 2.2l out of b-series pickups is the same as the 2.2l prope. Are the heads interchangable? Is one better than the other?

Yes the Probe 2.2l head will bolt right up, it uses the exact same block as the B2200. If your going to do it take your time and do it right there are a lot of little things and a few big things you will have to change. First of all when you rip off your old head check your valve reliefs in the pistons. The B2200 pistons had reversable Valve reliefs, stock I believe they are set 1 intake and 2 smaller exhaust. If so you will have to drop the pan and turn the rods around so you have 2 intake reliefs and 1 exhaust. Then next concern is that the distributor cap will not fit in then engine compartment. You can cut out the fire wall, try to find a mazda 12v cam that will fit or put an msd ignition in. I believe the head is the same as on the 12v 2.2l 626 and MX-6 so you might want to try that instead as you will probably have less problems with compatability. I saw a 12v 626 Turbo engine in a junkyard warehouse and it looked just like the probe. Right down to the valve cover which had a Mazda name plate intead of ford. If you try one of those the MX-6 probably has more performance. Either of the heads you use you will have to yank the entire valve train including the cover. The covers are interchangeable between the 12v heads but not with the 8v. If your truck is already fuel injected you should be ok with the intake, but may have to grab the intake that comes with the head. If your truck is carburated you may have to convert it to fuel injection. I don't know about mazda but ford redesigned the rocker arm shafts and how they mount on the journels. The 8v shafts are narrower and Mazda journels wrap completly around the shafts. The Ford probes journels have the tops cut off with a cap that bolts the hole thing down. The probes version is easier to take apart but is more likely to break. One other thing I noticed on the probe is that the Lobes on the cam are larger which reduces wear on the tappets and lifter. I havn't seen the Mazda valve train but overall you might consider a blend of parts.
Any questions about what I've covered here just drop me an email @ PyroHorniac@juno.com

I forgot to mention you will get an increase in power, although I don't know how much, I've heard anywhere from 5 to 20 hp depending on how modified your engine is.