Since you checked for air leaks & did a rebuild on the UIM/LIM, I would next think the MAF could cause the described problems. You cleaned it, so try unplugging it the next time you try to start the car. If the MAF is fubar, unplugging it will allow your car to run.

Next, since you have a donor car, remove the ECU from the problem 98 & put it in your car to see if the ECU is at fault. That way you don't risk your good ECU, it'll be sitting safely on the seat next to you.

I'll think some more on this ...

Must be that jumbly-wumbly thing happening again.