What you want is the least amount of fuselage cross-sectional area to contain what it has to carry.
For a slimer, that's the fuel tank. Find a long slim one.
The wing should be high-aspect ratio, semi-symmetrical, for a lower pitching moment. Lift comes from flight speed as well as wing area.
This electric weighs 3-1/2# empty. Intended to carry 3 sets of 10x3000 NiMh batteries, which weigh 69 oz. total.
A half-gallon of fuel takes about the same cross-sectional area with a long tank.
With 1324 in^2 on the wing, loading is a decent 9 #/ft^2. AR is 7.55 on 100 inch span.
Airfoil is an S7035, about 10% thick.
With a glow motor and that much gas it would be an all-day flier.
The controls are tuned for level flight and gentle turns. It is NOT an aerobat! And will never be flown as one.
Something to keep in mind.