The frames between the E-series and the F-series are different. That means that your fabricator's suggestion of simply just swapping the truck 4x4 onto the van chassis won't be possible without custom fabrication of parts. Are there lots of talented fabricators out there who can do this? Absolutely. Will they be able to do it in 70 hours and have the final product drive, handle, and be as reliable as an F-series? Probably not.
Lots of people have designed their own van conversions. Some have done it successfully and have awesome driving rigs. However, I think a good majority of these endeavors end up in 2 scenarios: 1) A converted rig that took twice (maybe thrice) as much time, effort, and money as originally estimated or 2) a successfully converted vehicle that suffers from lots of drivability issues.
Fortunately for you however, there are several companies out there that have done all of the work and research for you. A couple of these companies have successfully converted 100's of vans, but most importantly they have refined their systems by drawing on their vast experience of doing lots of conversions over the years and having lots of seat time and customer feedback. They also know what axles and from which years are compatible with what years of vans and can easily answer your question about going SRW or keeping the dually with the info about the pros/cons of each and what will be necessary to make it work.
A lot of these inquiries usually start out with someone thinking that they can complete a conversion for half of what one of the established companies charge. But in your situation it already sounds like you're okay with the idea that the cost of the conversion being around $12k. For that money you're in the ballpark of having a proven company do your conversion or getting an already established DIY kit and having your local guy install it.
My first 4x4 van was a one-off, homebrew. The welding was immaculate and the fab work was top notch. However, the van didn't ride and drive that great. It took hours and hours of driving, tweaking, driving, tweaking, re-doing, and tweaking until I got it dialed in. I think the converter was a great fab guy, but didn't have the hours behind the wheel of his work to get it dialed in properly. Successfully welding on some parts isn't the same as some very important design decisions like front end geometry, caster, spring rates, turning radius etc. How much time and money are you willing to throw at it with someone figuring it out on the fly?
If I were you, my first call would be to Ujoint Offroad. Not only do they have years of experience converting vans, they have also done tons of larger Class B's and Class C's. They also offer their conversion in a DIY kit form which you could have your fab guy install. You can also source your own axles with the Ujoint kit (they can tell you exactly what years are compatible) or you can order already rebuilt units from them (pricey, but completely rebuilt). They've already done the homework and research. I would at least give them a call.
Depending on where you're located, I would also look into Agile Offroad (no DIY kit though, installation has to be done by them) and Expovans. Each company installs a different front end suspension: Ujoint (Leaf Sprung), Agile (TTB) and Expovans (Coil Sprung). The knowledge these guys have about the van platform is astounding. All of these companies also stand behind their products and provide customer support.
A couple final considerations to think about:
Spending a lot of money to have a van that sucks to drive, doesn't perform up to expectations, or is simply unsafe, is a hard pill to swallow. Google or search "Javier Vans" for an example.
Expecting a smooth running 4x4 van at the end of 2 weeks of work time and not seeing the completed van for 6 months also sucks. This has happened to lots of people since it's not a bolt-on swap from an F-series.
Don't underestimate the time it will take your guy to research compatible parts, part #s, and design time to custom fab the parts necessary to complete a conversion on his own. All the little differences between the E-series and F-series will add up to a lot of extra time (trans conversion for 4x4, e-brake, ABS, brake lines, cross members etc) The major van converters have already done all of this work and research.
Resale of a reputable 4x4 conversion is higher. Their systems are tried, proven and well regarded at this point. Conversions are so expensive to undertake that a lot of people don't want to roll the dice on an unproven homebrew.
Whatever you decide to do, good luck. Be sure to start a build thread and post pics! The Chinook is an awesome platform.
