I just measured the seal groove on a 216 block I have on an engine stand in my shop. A 235 should be the same, as they use the same rope seal for both engines. The bottom of the groove is a few thousandths over 0.150", and the top is a bit over 1/4". The 1/8" shim should work about right. The way I've done the procedure in the past is to leave the used upper seal in place, lower the crankshaft a few thousandths by loosening all the main bearing bolts a couple of turns, and grind a slight taper to the end of the shim piece. Gently tap it into the block groove behind the upper seal. I use a pair of mini vise grips as a driver, clamping onto the shim close to the block surface and tapping on the back end of the grips with a small hammer. Repeat as required to get the shim into place without bending it. Replace the lower seal rope with a new one. It will probably be necessary to sneak up on the rear main cap bolts torque a little at a time while rocking the crankshaft back and forth as the seal rope compresses.
Jerry
