regular 40 gallon hot water tank BTU vary anywhere from 34,000 btu to
75,000 btu. the higher the BTU the better.

Because we ran out occasionally we went from a 34,000 BTU 40 gallon
tank to a 75,000 BTU 50 gallon tank. I wanted a 75 gallon tank but it
wouldnt fit the available space between toilet and furnace.

Tankless tends to be a hot topic here........ from inadquate heat, =
cool showers if you live in areas that freeze in the winter, no hot
water at all in a power failure, if the tankless uses line voltage to
operate, no hot water with valve just open a little, super expensive
install, needing new gas line and occasionally a new meter. teenagers
given unlimited hot water might live in the shower. expensive service,
tankless are complex and require occasional service, standby losses in
current tanks is actually low, and stanby losses help heat your home
in the winter, so most of the heat isnt really lost. life is full of
sytandby losses, tv, cable boxes clocks etc etc. anything that draws
power when not in use is a standby loss......

your old tank is probably full of sludge decreasing its hot water
ability.

a new tankless will cost a fortune.......... 3 to 4 times a standard
tank which is highly reliable.

so lets talk tanks

upgrade from your old 40 gallon probably 40,000 btu to a 75 or 80
gallon 75,000 btu will likely give you about 4 times or more your
current hot water capacity. and in its life probably never need
service

you double the gallon capacity and double the burners BTU rating,
roughly 4 times the hot water.

no more cold showers