Let's see:
48x12 55 = 576 sq. in. floor space
36x18 65 = 648 sq. in. floor space
The 65 not only gives you 10 more gallons (actually 12 because it's truly a 67 gallon tank), but also gives 72 sq. in. or 12% more floor space.
Total floor space and front to back is more important than length.
One FH would be fine in 65.
48x12 55 = 576 sq. in. floor space
36x18 65 = 648 sq. in. floor space
The 65 not only gives you 10 more gallons (actually 12 because it's truly a 67 gallon tank), but also gives 72 sq. in. or 12% more floor space.
Total floor space and front to back is more important than length.
One FH would be fine in 65.