The 8.1L in an Express van is extremely rare and is super reliable. I'd love to get a 8.1 camper because I like towing. If you change to 5.7, you will go back in technology to distributor and all moisture/misfire issues that come with it. Resale will be very hard. And you won't get much better...
1) Factory TPMS only supports 4 wheels, which two have you left out, or are you using aftermarket TPMS system?
2) Express van application dual axles never had ABS in them so I doubt there were sensor provisions. I presume the 11.5" axle you used came from a pickup truck. What year and model...
I have been contemplating doing a dually conversion to my Chevy van for quite a while. It seems that physically mounting it is a straight forward job, and any dually rear end from a 2003-current cutaway van will fit. Disconnect driveshaft, disconnect brake lines, disconnect parking brake cable...