Re: Rear tank mount and peacock questions
As long as the bottom of the tank is higher than the fuel inlet of the carburetor you'll be good to go. These are gravity flow systems, no fuel pump and fuel runs downhill only.
As for the petcock, the kit tank and kit supplied petcocks are a 10mm machine thread, not a tapered pipe thread. There should have been a red gasket, about dime sized that was meant to seal the kit petcock to the tank. It appears that you have replaced the kit valve with another type which I will guess is 1/8" male pipe thread.
If that's the case it will probably turn in a little, it should be tighter than finger tight anyway, to rotate the fuel line to where you need it. In your case I would have used a straight petcock instead of the 90 degree one you have which would have aimed the fuel line more directly toward the carb.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"