Bootless, the OHV rocker arms rotate around the rocker arm shaft. When you remove them, you are basically checking for cracks and wear. There are plugs on each end. Most guys will drill a hole in one cap and punch out the other side and then vice versa.
At $13-$20 for each rocker it's a lot of money for not that much performance enhancement on a stock engine with 300,000, in my opinion. Depending on compression and leakdown, I would replace the valve seats and do a valve job, using the original rocker arms. The valve seats tend to sink a few thousandths over time. I would then replace the lifters and put them back on. The bottom ends on the 4.0's are pretty stout and can last well past 300,000.
