To get full flow filtration, push the plunger IN until it bottoms, about 5/8" or so. It will be extremely difficult to move, and hammering on it with a big flat tipped punch and a BFH runs a distinct risk of cracking the block casting. I'd suggest drilling and tapping the plug, pulling it out of the block with a long bolt and a spacer, and either fabricating a new plug that's a very snug slip fit, or polishing the original plug a little. It would be wise to pull the plug out in any case, to do a very good job of cleaning out any sludge that has deposited itself in the passageways over several decades.
Jerry
