I would keep the temps constant ALL the time throughout the day and night. I say this because not too long ago my Kingsnake got an RI and I'm positive it was because the night temps got too low.

60F is DEFINITELY too low and will cause an RI. So, always keep the temps constant. UTHs are better since they provide belly heat, a lamp really isn't needed as they suck out the humidity in the enclosure which is important.

Also, the funny thing is, I highly doubt you'd have 3 feet of substrate the snake can burrow under to feel warmer. If you did, I doubt the snake would be warm, too. It would the same overall temp in the enclosure at night, so 3 feet of substrate would do nothing besides provide extra security if the snake is a burrower.