Massive oil leak five minutes after starting

After installing a head gasket, (all went very well). I started motor, ran it for ten minutes, stepped to front of truck when idle was good, and there was a 4’x4’ puddle of motor oil under the truck.
Because I poured a whole quart of oil on my cam and followers after installing the head, I automatically figured I had blown a rear main seal. Pulled transmission and replaced pressure plate/throw-out etc, while I was going to be down there anyways and all went well again.
Started truck five minutes into it starting again, same as before. A 2’x2’ puddle of oil all over my poor driveway. I took a closer look at where leak was coming from, and it seems to be above the starter/oil filter area. Is this an oil sending unit maybe? (Oil filter screws directly into side of block)