I ran into similar problems in the front of my bagged 710. I mini-tubbed the front to get the crossmember on the ground. I thought about raising the control arm pivots and shaving an inch off the bottom of the crossmember, but then the frame rails under the gas pedal were going to hit.
There isn't much "structure" in the back of a 510 to build off of. And I don't see a lot of room for a fuel tank. Have you thought about using the whole wagon floor-pan (all the way to the taillight panel)? Then you would have a beefier "frame" to modify.
I think they are Retro-spec flares: http://retrospec-datsun-510.blogspot.com/ but do some research before sending them money (I heard some complaints, and they might be out of business)
Ebay and Amazon take forever now … Summit is practically overnight delivery still .. (I don't know about fans, but I doubt they sell ebay/amazon quality)
Oh, and this wagon is awesome
You should measure the width of your windshield, because it looks like your roof was widened during the chop and the stock 620 glass might be too narrow now.