I used a sandwich thermostat that fits between the
oil filter and the side of the block. Be sure you have enough clearance at that point. Small block should fit. BB I don't know? Next, what is your sensor location? Side low in pan or front on small dip in pan? This will make about 10 degrees F. difference. In my opinion
oil isn't heated up until about 165 degrees and doesn't need to be cooled much before 200. Anywhere between the two should work just fine. I see about a 10 degree drop across my cooler (by IR measure).