I have only seen them clamped to the intake. This plate thingy sounds like something homemade. More than one way to do it.
Best way to do it would be to mount it on the frame rail straight accross from the ports on the block and a little lower, but the exhaust is usually going to keep you from doing this. Perhaps mounting it inside the left front wheel well and run the lines through the inner fender well could work. Considering where the factory put them, I don't think you could really do any worse.
Use the larger SBC filter if you do go with a spin-on. The bigger the filter, the less pressure drop you will have and therefore the more oil that will get filtered. Even in a full flow setup, most of the oil ends up being bypassed anyway.
