I'm no engineer. But I have an opininion (and just like everyone else I have one of those other things too). You said wishbones but it looks like hairpins to me. Hairpins have a little flex in them if they are not "bridged" together in the middle. Wishbones are traditionally mounted using truck tie rod ends and are quite rigid. Wishbones used to be mounted with spherical rod ends that are also quite rigid. More recently hairpins (and some split wishbones) have been mounted with rubber bushed ends. I think the combination of wishbones and rubber bushed ends should have enough give to work. Alan at Flatlanders in Newport VA (don't have the number right at hand) has been making and selling tube axles for years. Find the number and give him a call. I suspect he will tell you that your set up is OK.