Hardlikearmour, I've noticed that loaf pans vary in shape as well. Some have a more tapered trapezoid on the bottom while others are considerably wider (I am thinking of an 8x4 Kaiser pan and a 9x5 Cuisinart pan), so volume calculation would probably fail you here unless they are identical. Good thinking though, volume is the better way to estimate.

I'd bake 3 loaves. Assuming the height is the same (and lets say 3" for example's sake) you will have a volume of 270 with 2 9x5 pans & 288 with 3 8x4 pans. Versus a volume of 192 with 2 8x4 pans.

You could still bake two loafs in the pans you have, they will only be slightly higher on top. I think about 3 c flour is what goes into one 8x4 loaf, so depending on how many cups of flour your recipe calls for you can figure out if the dough will overflow or not. By overflowing I mean forming a sort of a mushroom cap when sliced, because you are woorking with yeast as opposed to a more runny cake batter. It's only an inch difference on each side, so don't worry.

Make two loaves in your existing pans and bake a roll on the side with the excess dough.