yes - I really struggled with the roof top air con and the solar panels - mind you I'm not superman either. I did experiment with 750n and felt it was too strong, so only replaced the back 2 with 600n gas struts. It is doable, but I purchased struts which were not as long as the originals.
The way I replaced them, was to prop up the top in 2 places on the corner which I was replacing. One on the back and one a little further along than the gas strut. Keep the props out of the way of the gas strut. When you undo the gas strut they do travel further out, so keep things clear. Another thing to be aware of is try and get your replacement with the same size ball, otherwise you have to replace the ball on the roof and also on the body of the van - one has a hidden nut! Then I attached the top end and using a long wood clamp gradually compress the strut down to fit onto the bottom ball. Protect the van from being scratched by the clamp by having some scrap along the top of the van. Then repeat for the other side.