If you got the engine installed and did the 30 minute cam break-in routine, that should mean that all is decently well with the "build" quality, I suspect.
IF when you let the rpm drop, THEN it wants to die, then that would indicate something's become awry in the carb's idle circuit, which richening the idle screw adjustment will not fix.
With the carb off the engine, when you're checking for the float issues mentioned, remove the venturis from the carb main body. When removed, the cluster should have some tubes hanging down from it, which sit in fuel from the float bowl (or a well connected to it). The small tubes are for the idle system, the larger ones with the holes in the side are for the main system. IF you did the 2000rpm break-in procedure, the main system is working.
For the smaller diameter tubes, there can be a drilled hole up inside that tube a little ways. That is termed the "low speed jet" in the carb section of the FSM. You might shoot some aerosol carb cleaner into it and liquid come out of the top of the cluster, but that doesn't mean it's totally open to specs! Take a bent-wire spark plug gap gauge and probe the tube, starting with the smallest wire and progress until resistance if felt with the larger wire gauge. Make sure the smaller wires "rattle around" in the rube. At some point, you might notice a little more resistance, which goes away with more pressure. Note that wire size. When you get to the wire that absolutely will not go, note that size too.
Then go to a hobby store and get a twist drill set. They usually have an assortment of sizes for the drills in the kit. Return to the carb's cluster and again probe the tube with the drill bits. When you get to the size that won't go, use the twist drill to enlarge the hole to that size, or the next largest size to "get brass". This will be the new size of the "low speed jet". Do each of the idle tubes. Spray cleaner through the holes for remove any brass from the "drilling" operations. Reinstall the cluster on the carb and the carb on the engine. The engine should idle reliably after that.
For more initial diagnostics, you can take some fine wire (like .010" diameter or so) and place a piece into the air bleeds on top of the cluster. These bleeds are an additional fuel curve calibration point. The smaller the hole/bleed, the richer the mixture will be, all other things being equal. Be sure to put a bend on the end of the wire not inserted into the bleed hole, to keep it from falling into the tube. You can start the engine and it will idle off of any residual carb cleaner in the idle tubes, but when that runs out, it'll die unless you raise the rpm enough to let the main system take over carb fuel metering. With that small wire in the idle bleed, fuel should drop into the air stream on that side of the cluster. Even with the restricted/clogged idle tube on the bottom of the cluster.
When I bought my '80 Newport 360 a good while back, we knew it would not idle once it came off the fast idle cam. As soon as it did, it died. Same if you backed out of the throttle running 60mph on the freeway and were slowing down for an off-ramp!
What I've described is how I finally got the engine to idle on its own. Both of the idle tubes had "accumulation" from fuel deposits in them. Enough so that idle fuel was compromised significantly. Idle mixture screw adjustments did nothing to help it, either. I looked in every Chrysler-related service manual I had, until I finally found an illustration of the idle/main tubes and where the "low speed jet" was indicated. Once I knew where to look, it helped greatly.
IF the timing chain was not installed exactly right, it could be "one tooth off". IF the engine runs better as you rotate the distributor to advance the timing by one wire's notch on the distributor cap, then it could be that the timing set is "one tooth off". KInd of a pain to deal with and fix. Ir aril won't run exactly right with the distributor turned, but it can indicate where the problem is, IF that's the issue.
I noticed you mentioned "overbore" in the subject line. A .030" overbore is pretty normal to get to fresh metal in the cylinder walls. It should have NO bearing on the issues you mentioned, by observation.
From my own experiences on my'80 Newport's BBD Carter 2bbl, you could well have something similar on your carb. Did this issue just start?
Keep us posted,
CBODY67
