Got a chance to open up the rear rocker box to replace the tappet guide and pushrod cover o-rings, adjust the pushrods, and replace the rocker box gaskets. Second verse same as the first (or front in this case).
Had to cut down a 3/16″ allen wrench so I could get at one of the rear rocker box cover bolts.
The tappet blocks are marked: RI for rear intake and RE for rear exhaust (the front ones are marked FI and FE).
Replaced the o-rings on the tappet guides. All four, front and rear, were leaking between the block and the case. This should fix that. Again used a little assembly lube to carefully install the o-rings without having them twist up.
Have some traveling to do for work, so it will be a couple weeks before I get to tinkering with anything else.