Oil filter access is pretty open on these cars, and I don't, at the moment, foresee romping on this build heavily enough to need an oil cooler. Olds engines actually overoil a bit- not so much on pressure, but on overall flow.
Yes, adapting an external mount is easy- besides a bolt-on plate, I've seen some racers just drill and tap the ports on the block directly. But up here, where a warm day barely cracks 60 degrees, and a cool early spring or fall day could dip to the low forties, it's possible to overcool the oil, keeping it too thick and hampering flow. (Which, counterintuitively, can increase internal temps.)
This build is going to be a budget streeter. The car itself needs a lot of work (rust, etc.) so there's no reason to go too far overboard. I'd been considering, for example, picking up a set of Edelbrock aluminum heads- especially since I didn't get any with this block- but they're $2,500 all by themselves, and a bit of overkill for a grocery getter.