Bushing lifter bores is a commonly recommended practice. The most frequently cited benefits of this blueprinting procedure are a reduction and/or elimination of 16 internal oil leaks and more accurate valve actuation since the lifters are now in their correct position relative to the cam lobes. I am curious to know if anyone has ever done a controlled test to determine just what horsepower gains can be expected from this procedure. I think it's obvious that just as with any blueprinting operation the benefits will depend to a large extent on just how far off from spec the item in question was to begin with. That said, what has been the average gain? My wild a** guess [ and that's all it is ] is probably from 1-5%. IF I am right about this than a 500 horsepower engine would gain anywhere from 5-25 horsepower. Also what is the longevity of the bushings? I would think it would be somewhat longer if the bushings had small oil holes, say .040 drilled in them. I would be inclined to do this anyway since it would allow for a greater variety of lifters and pushrod oiling should it be desired.

if you have the opportunity to get your block on a CNC machine there are tools that measure everything and are able to generate pretty conclusive reports.

Deck heights in all planes
bore spacing and center to center offsets in both x and y direction
lifter bore spacing and offsets

there are countless measurements it can take with a Reinshaw (sp?) probe.

I can't speak about gains, but the safety margin if a lifter pops out is a good side benefit to getting things right. I am sure you are on the right track on possible gains, as so much depends on just how much is out of wack to start with. Hoe close to optimum the cam is to start with would make a big difference I would think.
I have bushed lifter bores, with a .060 hole, big enough to feed the lifters for oiling even with lifter oil holes 90 degrees to the hole.

I remember back in the day that Bob Glidden claimed he was one of the first to incorporate bushing All of the lifter bores to correct geometry for a hp gain. He claimed something like 25 hp+ depending how far things were off to begin with.

Nowadays it is becoming more of a common practice for some. There are still Plenty of us that dont though. I wanted to on my recent engine but skipped it. Though I myself recommend it.

Any machine shop owners/engine builders care to comment? Curious minds want to know. Actual dyno #'s comparing before and after greatly appreciated.

I have bushed about 80 blocks, and have found some blocks dead nuts, and some out of line as much as .025. One of the very worst was a 1964 Hemi block. Never done any back to back testing, but it gotta help on the bad ones.

Fast rate cams are very sensitive to accurate lifter bores. The '03 Team Moparts Engine Master build had some issues and it caused a S__t storm with Hughes. Here is the thread buried in the Tech Archive due to it being 0ver 10 years old.