Max, As Neil has described, it’s at the front most extreme part of the RH side of the cylinder block.However, It can be prone to disappear when the block is resurfaced on the mill.There is occasionally another point where the number is stamped, certainly on the very early 1949/50 blocks, and that is on the top of the bellhousing area of the block, about 2″ to the rear of the distributor hole, on a small flat area integral with a casting rib. As it’s not a milled surface, the numbers are stamped straight onto the surface of the casting.As a matter of interest, many of these early blocks don’t always follow the normal/later numbering practice as described by Neil, but have just a short letter+number combo, such as 6M55 or 6M183.Certainly some of the earliest works Cadillac engines for J2 and,I believe, JRs had these.The later 365/390 blocks, I believe, also have an engine number stamped on a machined surface on the lower, left hand side of the block in the area of the dipstck tube base/sump flange.I’m hoping your car doesn’t have one of these!Regarding your oil leaks.It may be prudent to extend the dipstick tube to the top of the block.Will require an appropriately longer dipstick!This should alleviate overspill especially on hard right hand corners.The ‘normal’ oil breathing arrangement of a ‘road draught’ tube at the back of the lifter valley cover, can be utilised for purposes of MSA spillage requirements by shortening the tube, rotatating to point at the gap betwix the rear of the LH inlet manifold and distributor and fed into a catch tank on the LHS of the engine bay.Likewise, you may wish to replace the oil filler/breather cap arrangement, by having a screw on cap without a hole, and letting in a breather pipe to also end up at the catch tank.
Hope this helps
Roger