I've got an Atlas and an Electra. Here are some rough measurements:
Atlas - 8.25" at the rear, 13" at the widest point
Electra - 7.75" at the rear, 11.5" at the widest point
Those measurements are pretty close. The Electra is in pieces, so measuring the tank was easy, but I didn't feel like taking the Atlas tank completely off to get exact measurements since I ride it everyday.
And the Electra tank isn't as long as the Atlas.
If you're considering putting an Electra tank on an Atlas, I'd advise against it. Only because it would be a lot of work to make it fit. It's already not big enough to get over the featherbed frame rails, and the tunnel and front mount on the Electra tank most likely won't clear the head steady on the Atlas. Also, the Electra tank doesn't drape over the frame like an Atlas tank.
And then you'd have the issue of a big gap between the rear of the tank and the front of the seat. Unless perhaps you have a Manxman seat.
Basically, I think it'd be more trouble than it's worth to try and adapt an Electra tank to fit a featherbed. Atlas tanks pop up on ebay pretty regularly, and they can be had for a pretty good price.
Hope this helps. Feel free to PM me if you need more info.
Thanks
Jeff