I wouldn't expect a lot better fuel economy with a 454. You'll definitely get more power. I don't know about swapping in a 120,000 mile engine. Do you think you'll have an opportunity to freshen the 454 up before dropping it in? It seems like, if you're going to go through the trouble of making the swap, a newer engine would be nice.
If you're going to tow with a 454, the 4L60 probably won't hold up, even with a shift kit. Sooner or later, that sun shell gear will come apart and waste that whole transmission. Try to get the 4L80E and cross member from the donor.