maybe i cant read rpos or have a wrong list of some sort but i swear its a 502 based on the one i have.All the medium duty trucks that were factory propane that I have seen had the 454. Gm put in the 8.1L in 2001+.
All it would take to get it to run would be to put an older hei distributor in it. You would then need to make sure the 425(propane carb) has the right airvalve in it. With the correct airvalve and HEI distributor it will run without a computer. You will need to do some research on which flexplate will work with the bb/700.
The factory computer shuts em down around 3800 rpms.