1989 TBI 350 swapped in place of 1989 TBI 454- ECM question

I have removed the TBI 454 from a 1989 Chev K3500 and replaced it with a TBI 350 from a 1989 V1500 suburban. I know anybody seeing this will ask why before anything. The owner of the truck is "hoping" to get a little better fuel mileage. The truck has a TH400 transmission in it and the suburban has a 700R4. I didn't take into consideration that the ECM may be controlling the shifting or at least the downshifting on the TH400. Both ECM's have the same service ID #'s on them but I still used the ECM from the 350 and wired in the ESC module. Now the truck pretty consistently downshifts while driving in 3rd gear. My question is, should I swap ECM's so that the original one from the 454/TH400 is back in there? If so, how does the ESC module play into things? Will it run ok without it? I'm having trouble working this through in my head.
Thank you.

The ESC module controls timing from the computer and also acts as the knock sensing module hence electronic spark control module. With the 454 pcm and the 350 pcm being the same service number simply means the pcm itself is the same. You would just change over the 28 or 24 pin EPROM from one to the other. The electronic trans was introduced in the truck chassis in late 92 for the 93 model year which does not use an external ESC. The first true pcm was the 8625 service code for trucks. Cars had electronics trans as early as 90 I believe. So use the 350 chip. You won't get any better mileage. My 93 4.3 l gets less than the 350 in the same model except I have lean burn enabled on mine.

Also 700R4 is non electronic except for lockup. TH400 is the same.