Something else to consider, Piston slap is usually worse when cold and goes away when the pistons get up to temp and expand. I have never heard piston slap get worse as the temp rises, sounds more like a bearing / clearance issue to me.
Your piston is definitely not hitting your cylinder head.
If you replace the engine, remember that a new / reman engine would likely cost every bit as much as the car would be worth in good running condition, not to mention the cost to install. I would just go with a good used engine and call it a day.
1982 Cutlass Supreme - mild SBC, tons of fun! The summer toy
1996 Silverado Z71 - the hauler
2002 Bonneville SLE - His / project
2003 Bonneville SSEi - Hers / never ending project black mirror