If you lower the tank, the engine will run richer when inverted. If you raise the tank, the engine will run richer when upright.
Like Rusty wrote, the ugly end of run (on-off-on-off) can be fixed by shimming the back of the tank outward. But it can also be fixed by moving the LO's forward. I'd shim the tank 1/8"-1/4"and then work with the LO's to get them correct later. Leave the tank aft end shimmed out, it won't hurt a thing. Only watched the first video to this point.
If the engine is new, I'd run less prop...the 11-4 APC or 11.5-4 APC would work. Have you got plenty of oil in the fuel for the newer engine? I ran my first .46LA one tank full in the plane on the ground, saw that it was stable, and flew it on Powermaster 10-29 GMA. After that gallon was gone, I change to 10-22.
I'll go watch the other video...ah, the second engine is going lean in the outsides, so lower that tank some more. I kinda like to make my first adjustment 'coarse', so I can see something happening, then perhaps work backwards toward where it had been. I just don't see any sense in making a tiny adjustment a dozen times, when I can make a big adjustment and get something done right now! I'm sure opinions will vary.
Good
wide, free turning tires will help a lot on grass. The stock Brodak wheels should be good.
Steve