Here's what I can tell you out of my general car knowledge.
#1 - your transmission will NOT work, but, it has nothing to do with the horsepower level. The iron duke 151 shared the same "60 degree v6" bellhousing pattern of the 2.8 v6 engine... this is different than the standard SBC pattern. By the time you bought atapter setups,l you'd be ahead to get a stronger transmission to start with.
#2 - your iron duke engine was fuel injected. You need to decide whether you are going to run a fuel injection set up of some sort (TBI, TPI?) or a carb, and if a carb, are you going mechanical or electronic controlled. No, the plugs on your TBI harness are NOT the same as what would be needed to plug into the v8 TBI. Regardless you are looking at a different engine wire harness, because the plugs and lengths need to run to different places, the engines are different sizes, etc.
#3 - I have a little info on the motor mounts, a starting place to research a bit: I've heard the 151 had its motor mounts in the same place as a 301 would (that it was designed as one cylinder bank of a 301 similar to how a chevy 4.3 is 3/4 of the length of a sbc) In theory, you SHOULD be able to drop any pontiac v8 whether it is 301/350/400/455 into the same mounts and just put a new tranny/harness in around it. What I do *not* know, and I *assume* to be different, is that the sbc and pontiac v8 use different mounting holes on the crossmember for the engine pads. if you had your engine out, and a junk sbc block for mockup lying around, you could measure and see if they are the same.
Something you didnt mention, but I assume you know, is you likely need to upgrade your fuelpump in the tank as well, BUT eliminating the computer should mean going to a mechanical pump and changing that sending unit on the tank from a electric pump to a mechanical (if the computer is eliminated, nothing there to tell the pump to turn on).
I've got a 85 bird here I'd been trying to sell for a while and pick up a duke car thats nearby, but, no takers in this economy so I've researched the reverse of what you are and am looking at ripping out my sbc and converting to the 151