Hello and welcome to the forums.
Yes the ideal orientation is for front to be intake and the top to be exhaust as that's how heat convection works.
However in your scenario you'll have negative air pressure. It's not a big deal but you'll be pulling in more dust from outside than you would compared to if you had equal or positive air pressure.
Usually to equal the air pressure, you put a fan on the back fan mount of your case to be an intake/blow in. But again in your scenario, i dont think it would fit in between that rad.