I'm sure it's not the issue,but getting guides knurled isn't going to be a first class workshop practice for most... What was the installed spring heights and pressures? If there wasn't enough pressure on the seat the lifters may actually pump up slightly and the valve may be kept off the seat slightly.

If I recall right, there are 2 large cooling passages in the back, right by #6 ex.

To Steve's point I gotta wonder if #6 ex lifter has a leak down issue....like it won't leak down properly or has a problem with the retaining clip. I would probably replace #6 ex lifter with a new one at this point with 2 burned valves at that location, just in case. Everthing in that valve's train is suspect at this point. Even a weak spring.....the stock ones are pretty light for that era. It is cheap to replace them all and eliminate any weak springs; there are a couple of stock replacement PN's in the thread on max valve lift.

More likely the lifter is pumping up with a low on the seat spring pressure ...it's not uncommon to shim the springs to get the correct on the seat pressure after the seats have been cut a few times.... If it's been aroblem on that specific valve then maybe check the lifter and take a look at the lobe as well....if it's wiped a lobe and only lifting the valve a limited amount it will burn a seat...worth a look anyways !

Did you look carefully at the manifold gasket areound exhasut port #6 when you removed it to see if there were any leaks? A big enough leak in the exhaust close to the head can burn a valve (with the oxygen getting back in at the hot valve). Ditto for any crank in the exhaust manifold.

if not running lean where is the total timing set at?