Sorry I don't know what I hiab crane is... I know I would have to fab something to do lifting at my place to attempt what you're doing. I have built several fire brush trucks in my "Shop" (It is a carport and a gravel driveway.) The last two were built out of M923 Military 5 ton cargo trucks. The biggest issue is wire feed welding in the wind. I used the old reliable "Cherry Picker" for lifting the pumps and engines onto the decks. Tanks were installed using ingenuity and levers made from wood.
As far as your 6-71 leaking oil having worked for a Detroit Diesel Distributorship for all of the 20+ years, if you put them together properly the only "Oil leaks" are supposed to be the air box drains. You can out a collector on those to keep it from dripping on the ground. If the drains are not open the oil can fill up and cause the engine to over-speed from a secondary fuel source being the oil. (Diesel engines don't runaway, the engines over-speed. The mechanics runaway, at least the ones that don't know what they're doing.)
The main thing that causes oil leaks is people use sealants on the gaskets. The gaskets are designed to be installed on clean dry surfaces, use guide studs on vertically hanging gaskets and use proper thread sealants and copper washers where directed on bolts in the service manual. I personally built a lot of engines and years later were still not leaking oil.