Unless you have a pumped/regulated engine I do not recommend placing the tank as far back as you suggest. You will have troubles adjusting the engine to run smoothly both when climbing and diving. Mount your tank in the position suggested in the plans

As for long fuel line lengths, the friction between the fuel and the inner wall of the fuel tubing increases linearly with length and decrease with the internal diameter to the fourth power, i.e. decresing the i.d. to half the original value will cause the friction losses to increase 16 times (2x2x2x2). Thus it is much more important to use large i.d. fuel tubing rather than keeping its length short. The distance between the tank and the engine should be kept as short as possible because of the pressure head differences.

/Red B.