Back to the fuel thing, agree that selector looks unique for sure. But does it work? If it works, and you said you verified it’s switched to the tank you want to use initially, I’d just fill it with gas and send it!
Regarding the off side tank, if the fuel pickup and filter sock from the tank you removed was ok, I’d presume the passenger side is similar condition.
As said, siphon or pump out as much old diesel as possible and put gas in it. Or just wait til you have more time and drop that tank and empty/clean it and run off your new tank only for now.
Kind of depends if both senders and switching valve work or not imo.
Fwiw we once proved that a Mercury marine V6 will run on approximately 2/3 gas 1/3 diesel. Back in the day a dock helper filled his boat with diesel at the fuel dock. No one knew it until the boat died. We were on vacation and time was of the essence so we towed it back to the resort, put it on the trailer, siphoned as much diesel out as we could. Guessed it was 2/3-3/4 of the tank. Topped it off with premium and it ran. Smoked like hell the first day. Topped off with gas again, less smoke then, but it did no damage other than the weirdest exhaust smell ever and the fact it was a mosquito fogger for a couple days.
