650ss Head Damage ??

Are you saying you don't have a blown head gasket?  It being new certainly doesn't mean it isn't leaking.

Did you re-torque the head after the first time it got hot, then cooled?

You could always try JB weld as it is only an oil leak.
Put some weld over the bad area, grease/oil the barrel over the same place as the bad area on the head. Replace head on barrel without gaskets, leave for a few hours, should still be soft, lift head off, remove any JB from unwanted areas smooth flat once hardened off.
As for the loss of compression you will need to investigate the valve seats, valve faces piston rings, cylinder bores, and tappet clearances to find the culprit.
At a guess I reckon your tappets are not set correctly. 

Do not skim the head.  Doing so will cause knock-on issues with the barrel spigots, gasket sealing, pushrod lengths and oil leaks.

Your problem could be quite simply that the head has been already skimmed and the spigots are holding the head off the gasket. You can get extra thick solid copper head gaskets but they are not easy to anneal correctly.
Measure up and work out if your gasket is doing its job properly.
Try a composite gasket.

I made a sandwich of a solid copper and composite gasket to prove that my spigots were too long.
Many people will tell you that the Ethanol in today's petrol will affected JB Weld to the point where it makes it goes soft.

very easy to missplace a pushrod which will give a loss of compression. , usually need to check bolt tightness after first few miles ,then again after a 50 mile gentle run . Also found no valve clearance after someone missread the proceedure to set the engine position.Would not think the damage relevant.

Torque sequences can be made irrelevant if you tighten the bolts down gradually a bit  at a time until you finally come to the actual required setting, this way any mis sequencing can be of little consequence in the scheme of things.
As for the JB stuff, it does of coarse desolve in ethanol but in this case it wont be a problem to my mind being where it will be situated.
Every thing to gain including the experience of seeing your actions work either way.
 

You mention the problem was near the rear.  Is that awkward bottom centre nut tight?  Mine stripped years ago and was helicoiled 
JB or similar should be worth a try.  The surface we can see looks good.  Have you cleaned it after the leak?  It's very clean...doesn't look like gas has been going through.
Is there an oil leak from a rocker cover?  It  can ooze round the joint and come out at the rear.

I discovered as follows in the DSNs Oil Leaks section.............

The early 650 and 750 cylinder heads, made at Plumstead, were cast using a very coarse alloy mix which led to porosity issues. These often appearing in the vicinity of the back right-hand side head bolt. This being next to the rear oil drain tunnel which itself gets the full force of any internal crankcase pressure. The leak can be due to oil forced through the gasket in this area or even the metal itself. Back in the 1960s, a temporary fix for the weeping metal was a good clean of the area and then a coating of Araldite warmed with a gas torch to help it get into the pores of the metal.

The photographs show the rear of the cylinder head at the point mentioned above. The drain hole leads from the rear rocker box down to the crankcase. This it does when the pistons are rising. But when falling the pistons increase the pressure within the crankcases. This is used to force oil and oil mist around the engine to lubricate the camshaft, follows, pinions, gears and chains. The camshaft breather takes away a good percentage of the pressure but as well as lubricating the internals oil will get pushed up from the sump, through the drain tunnel and back to the inlet rocker box. On its way, it has to pass through any cylinder head gasket. This is the weak area mentioned above where leaks can occur.