Check the instructions, some heaters require a certain orientation, some do not.
If you want 78 degree water and your house goes down to 60, one 300 may be a bit light, at lot depends on the top and lights -- open vs. closed top (evaporative cooling), type of light and how long (e.g. florescents in a hood heat a lot more than LED
's hung above).
The heat loss from conduction through the glass is directly proportional to the temperature difference. So a 78 degree tank in 74 degree room needs a quarter the heat (for conductive loss) as a 78 degree tank in a 62 degree room.
I agree with the comments above about 2 heaters, there's a lot to be said for that -- if one fails-cold you have at least some backup, and if one fails-hot, the smaller wattage will take a lot longer to kill your fish.