Need some help to make sure I don't throw money down a rabbit hole unnecessarily.

I just bought a 1998 Astro with a 5.7 Vortech from a Savanna van. The PO claimed the motor was from a 99, but the PCM in the van seems to be from a 97 truck. The Black box serv.no. is 16244210.

Here's my problem...The truck starts right up, drives through the RPM range just fine, but it idles at 1000-1500 rpm! It'll eventually drop to about 750, but if I touch the gas again, it goes back up and stays there for an undermined amount of time. Also, the tranny doesn't shift like it should, the other day it slipped like crazy ( very cold out ) and once I had driven a few miles and stopped for coffee and came out, it was shifting again. The 1-2 shift is decent, the 3rd gear seems soft and it drops into 4th heavily. I don't think it's a tranny issue...he said it was rebuilt 10k ago and there is good pressure, it's solid when it's taken out of Park and put into gear, forward and reverse.

I've been doing a lot of reading here and it seems to me that there might be a harness issue since there supposedly was a change from 97 to 98? I had a p1406 code with freeze frame data stored, as well as p0122, p0327 and p0420-30 codes set originally, but when I removed the codes with my scanner, only the EGR Pintle position code showed up again. I pulled the EGR and replaced it as the plunger seemed to stick a little ( found a nice one in the local yard that operated smoothly and in much better physical condition with a newish looking gasket under it ( assuming it was a replacement part not much before the demise of the van it was on )

I replaced the TPS as my scanner was showing 0 - 1.6 fluctuation and pulled the IAC valve to check for blockage/carbon and operation. I plugged the IAC into the harness and turned the key on to check operation, the motor moves the valve smoothly, so I don't think the IAC is causing the high idle. My next step is to do a vacuum test now that I've verified the other components.

HOWEVER, I'm stuck on the idea that this is a computer issue. The local go fast shop wants $275 to do an off dyno tune. He says as long as it has a v8 PCM, he can do it. Before I even think of shelling out that kind of money, I want to make sure I'm providing a good canvas on which to tune. I don't have the ability right now to go crazy and do a loom switch to a 411 PCM ( which I can get for 50 bucks at the local yard ), but I'm wondering about the 97 that is in there now. I wonder if it isn't cause for some of the problems there is with general driveability and if I would be better served updated the PCM to a 98-00 PCM. He claimed to have just done the extra wiring for the extra injectors, so I assume he didn't do a wiring swap with the motor, just used the existing from the 4.3. Should I expect a 98-00 PCM to be more plug and play with the motor/tranny or should I just buck up and go see the speed shop?

My driveability isn't what it should be, and I'm getting 12mpg. I know this set up is good for much better MPG.

Also, the transfer case is a pushbutton 4x4, no AWD.

I'd certainly appreciate anyone who has any advice...especially if you went through the trouble to read my little diatribe!