My take would be to print vertically, but on supports, instead directly on the build platform, because of all the vertically oriented tubular sections on the base. Preform might try to give you a bunch more supports than you need or the top area; all you really need there are a decent number of support touch points on the flat faces that are facing straight down, the, uh, bosses(?) around the bolt holes (?). If you need the bottom end to be really flat, you might need to pad that end model with an extra millimeter or so, not to deal with Z compression (since you’ll be on supports), but so that you can sand away the rounded surface you’ll get from printing a flat face parallel to the build platform.
It also may depend on what your goals or the print/part are - fidelity of the internal structures, straightness of the tube, parallelism of the end faces/flanges, circularity of the bore? Are you having trouble printing the parts at the angled orientation, or do you just want to find an orientation that prints better and/or uses less support material?