Engine dies when idle

I did a 4.3 GM swap, relatively easy. No alterations required.

other than mounts n what not right? how about the tran?

I used an automatic, four speed, even original Mazda shifter!
I made motor/transmission mounts, used original engine mount pads in the truck, same radiator same throttle cable!

2600 use different motor mounts, on the frame, than the 2000/2200.
Please clarify which chassis you did the swap in, for those who don't already know.

B2600i, but isn't the frame the same? I unbolted the 2600 mounts from the frame, bolted the v6 mounts to the same holes used by the 2600 engine mounts, I am telling you, drop in fit, engine came from truck, not a van. Things mount too high in a van.

Yes, the frame is the same, but the mounts that are welded to the frame are different.
Here are the 2000/2200 mounts. It has almost a 45

Okay, but '60's through '80's mounts will work on those angled mounts edited add >, matter of fact, the engine can sit lower on the angled mounts. You just make a "U" shaped bracket to straddle them, bolt goes through to hold mount. It bolts to the original pad. Drill holes for the two bolts. When sure engine is where you want it, can weld in place.
Make cross member for transmission mount.
You will have to modify the cross member under the engine to fit around the oil pan.
Then it mostly is hook stuff up. Power steering pump is right there beside the steering sector.
edited add > Will need to modify radiator or, as I did, make up lower radiator hose (I used a piece of pipe crossing under the engine to the lower hose inlet on the radiator).