what do i do
 

You're probably on a good track.

The Electra's 550 motor is 15 year old tech, and the modern 400 with gear drive will easilly outperform it. With a modern lightweight speed control and elimination of the RX battery (which wasn't safe when I had an Electra, it was that long ago...) And using NiMh batteries, you should be able to have a 1200 mah 7 cell pack and still save a LOT of weight vs the expected 1200 mah old style R/C car type pack.

The weight savings, and increased prop efficiency should give a dramatic climb rate improvement. Glide ratio will improve also if you use a folding prop. Because you should climb faster.... you should get a major increase in flight durration per charge.

Glide ratio (L/D, not durration which gets into a bunch more variables' relationships) is not as sensitive to weight as the rate of climb.

Unfortunately... I'm not famliar with which props are appropriate for the geared 400... you are looking for a power prop, not a speed prop. Long low pitch in the range for your drive. The Electra won't fly very fast so a short high pitch prop is futile.

what do i do
 

Not knowing the wing area and stall speed of the plane, your question is kinda moot.

However, start with a 9x6 e-style and go from there.

VP