Hello, a bit of a long post, but I'd rather include too many details than not enough...
I'm the original owner of a '78 E150 base van, 300 CID engine, currently approx 150,000 miles.
Except for a few normal wear&tear and mainenance items, it has been extremely reliable over the years, and I intend to drive it as long as it holds out. The engine runs fine, and still starts up as easy as the day I bought it.

The past few months I've noticed a bit of a loud rattling sound from the engine at high rpm under load (when shifting gears, uphill, highway speed, etc).
A few days ago I decided to try my hand at checking the valve adjustment with the engine idling, and noticed a regular tapping sound coming from one valve. Turns out the lifter in that valve had give to it with the engine off and that cylinder at TDC, so I replaced the lifter.
Even though all of the other lifters seem fine, I'm tempted to replace all of them while I have everything disassembled. Push rods and rocker arms are all fine, so all I would do is replace just the lifters.
Also, I pulled a few of the good lifters to check for any dishing on the bottom, and all are perfectly flat, so I'm guessing the camshaft is fine.
Factory specs say to turn the rocker arm nut one full turn after zero lash, but the most I can turn any of mine before bottoming is between 1/2 to 3/4 turn, which I'm guessing is not a problem. I turned them all to just beyond 1/2 turn so none of them bottom.

The only lifters available for that engine these days are aftermarket ones. Locally about the only ones I'm able to get are the 'Sealed Power' HT900 (hopefully not a problem to specify the brand here), which I used to replace the failed original lifter.

Should I leave well enough alone and leave the rest of the lifters in place, or should I replace all of them while I have everything apart?

If I replace them all, are the aftermarket ones reliable enough that I shouldn't worry about one of them being defective right out of the box?

Since the valve clearance check requires completely bleeding the lifters, I'm guessing new lifters don't need to be primed before installation?

TIA for help with this!

 

OK, apparently I didn't check through the forum thouroughly enough before posting the above in this 'van' section ... just found the 'inline six' section, and have moved the post there, along with an update