Getting a new tank - what shape?

My three used-to-be-small goldfish, Comet, Goldy and Mina, have
outgrown their 10 gallon tank and are demanding new accomodations.
(And yes, so far their water quality has been good with lots of
testing and water changes.)

I am thinking of getting a tank between 30 and 40 gallons and I'm not
planning on adding any more fish. It will be sitting on an industrial
strength lateral filing cabinet that is 36" lng and 18" deep front to
back. So those are my "footprint" limitations.

Now I am wondering what is the best shape. I know that the largest
surface are of water will give the best oxygen transfer.

My tank will have either sand (which I have now and like) or a fine
gravel bottom and will be planted (if I can keep the plants growing
ahead of the goldfish eating). So I will need some depth front to back
for the plants and also sufficient water depth for tall plants.

I have my eye on a nice 40 gallon that is 36" by 18" byt about 14" or
so high (not sure about the height but that would equal 40 gallons).
I'm a bit concerned about the weight - it would be something like 330
pounds!

Any suggestions about what shape tank goldfish like? Tall, flat, wide?

Also, any recommendations on the filter to get?

Thanks. I have learned a LOT from you guys over the last few months!

Judy