An LS swap into a 715 has been done numerous times so its nothing new.
Wiring harnesses that make the swap plug and play are readily available. Since you have the entire van... you'll have the engine ECM, trans PCM, Gas pedal, and pedal ECM already with the van... so should be able to just send it out to get security stuff removed. (I used LT1swaps.com... simple and cheap)
You'll need to rework the fuel tank to have a fuel return also and put in an inline high pressure fuel pump and regulator/return on the frame rail somewhere.
Good luck.... I had a 5.3L LM7 / 4L60e combo in my 1966 Chevy C-10... it was a RIOT and was VERY happy with the swap.
67' M715/5.9 Cummins/ZF5/NP205