How easy it is going to be depends on how good of a mechanic and fabricator you are. Projects that some find to be simple might look daunting and impossible to others. Body work on vehicles always seemed like something way out of my league but after tearing into a vehicle and replacing cab mounts and body supports I now realize it’s easy. As long as you have the patience anything is possible.
As far as the swap none of your electrical from the van will work unless your HMMWV has been converted to 12v. Glow plugs will need swapped which is simple. Is the van motor full mechanical or fuel injected? I would think if the van motor is full mechanical you should be able to swap the heads, exhaust, fuel pump and other related parts. There will be little things like throttle linkage and kick down cable that will be different.