So explain how conventional hydraulic lifters work in a 225. What's the problem with explaining and increasing everyone's understanding?

I just did. The slant six has conventional lifters. There is nothing special about them.

I have no direct experience with hydraulic lifters of any kind, but I have always wondered how effectively the hydro slants manage to pump them up from the top. Seems like you're already starting with lower oil pressure (from the head), and then there's a lot of sliding joints for potential leakage (shaft to rocker, rocker to pushrod, pushrod to lifter) that would reduce the available pressure at the lifter.

Is the lack of a side hole the only difference between Mopar V8/AMC lifters and Slant Six lifters, or are there also differences on the inside? Do the V8/AMC lifters also have a hole in the top?

Just playing Devils' Advocate here... IF the valving is the same, AND the V8/AMC lifters have a top hole, then the only issue I see using them in a Slant is the addition of another big leak path (side hole to lifter bore). How badly that affects performance is way outside my wheelhouse, though.

I don't know how many readers really understand how a hydraulic lifter works. The amount of oil pressure to the lifter really not important. the oil supply to the lifter is just to make up for "leakage". If the lifter is full of oil, and there is no leakage, no additional oil is necessary. Of course, there is always some leakage, so some replacement oil is required, but pressure requirements are minimal. Most engines with hydro lifters oil from the oil galley, to the side of the lifter. Then oil the valve train thru the pushrod (major leakage), so a fair amount of "volume" is required, at enough pressure to force the oil up to the valve train. Since the slant oils from the pushrod down, the oil requirements are much less. I don't know if any one has ever measured the oil pressure at the top of the lifter, but I would bet it is only 1 or 2 psi. The oil in the lifter does not raise the internal plunger (spring does that), the oil cannot be compressed, and in effect makes the lifter solid. Did I get that clear as mud?

Just for some useless to the solution to the thread slant trivial information per the Slant Six History by Mr. Weertman:

The Aluminum blocks had provisions for the hydraulic lifter conversions, but the engineers objected to 12 cross drills
that might cause future leaks...in 1967 when they were working up the 246 CID slant six, they were going to add a second
galley for the typical hydraulic lifters used in the other platforms. Ten years later a Canadian engineer worked up the
fed from the pushrod tappet design...and it met a lot of skepticism that the design would actually work...