Weight & Balance and Fuel Capacity?

A 415C has a maximum gross weight of 1260 pounds but it can be increased to 1320 via STC. Empty weight is different for every airplane. There must be a weight and balance sheet in the airplane to be legal and it would have that airplane’s empty weight. As for fuel capacities, earlier models had 5 gallon cowl tanks and later ones were 6. All post war Ercoupes had a tank in each wing. Some were 8 gallons and some were 9. Most are 9. So that airplane has at least 5+8+8=21 gallon capacity and possibly 6+9+9=24 gallons.
By the way, I doubt if your empty weight is anywhere near 749. More like 947! Our planes have gotten heavier with interiors, instruments, etc. I had an artificial horizon in mine that weighed 12 pounds and barely worked!