I've loaded lots of 16 foot sailing dinghies into containers for transcontinental trips. I also brought an RV-4 (HAh!!!) in from Idaho in a 20 footer.
If at all possible get the container off the truck - it makes life MUCH easier. (Some trucks have the capability of moving the container off sideways).
If you have to use a forklift be (obviously) very careful about where the lifting force is applied (crush damage to rear fuselage......)
You know about the tailspring mount issue
- you will have to take the tailplane off I'm pretty sure. If you take the tailspring itself off then lots of padding and maybe a little custom mount to secure the back end safely? Screwed to the floor. Also wheel chocks screwed to the floor?
Assume that the container will be slung around a lot, probably dropped, etc etc. Get everything very well secured so it cannot possibly move, but remember that the straps/rope or whatever can also be a source of damage.
There will be a goodly number of tie-down points within the container, but you will probably find most of them are not quite in the right place. With the boats we made custom-frames that wedged in top. bottom and sides, which worked really well - never had any damage.
I'd also drain all fluids - drain the tanks fully. I'd also drain the brake fluid. Not sure about engine oil - the inside of the container will be hot and very humid....
Can't believe you are selling that lovely RV4 you've done so much work on!!!!
Cheers
Chris