Cool I did a similar thing with second edition except capping characters at level nine. It worked fine mostly. I tried the xp cap at 300,000 but it didn’t quite work as the Druid xp table is kind of… odd. My solution was either a hard cap at level nine or making Druids use the cleric xp table.I took a good long look at replicating E6 in 1E. Doing it for 2E would be about the same. Problem is that 3E is structured differently and helps tremendously to make E6 actually work. PC's stop advancing at 6th level but get regular improvements in the form of feats, while their skills improvement basically halts. There are almost no other changes that need to be made to the rest of the 3E rules. That just doesn't hold true for 1E/2E. For starters the xp table is different for every class and there isn't a single level that makes a good stopping point for all of them. Beyond that, there isn't anything like feats in 1E/2E to continue to reward PC's with. Closest you could get would be NWP points but they make a poor substitute and I felt they'd start to imbalance the game in ways that feats wouldn't.
What I DID do was to come up with a variation for 1E where all PC's cease advancing entirely at a total of 700,001 xp. That puts different classes at different levels but makes a good stopping point nonetheless as the PC's are fairly comparable at that point despite the level differences. I then made some tweaks to the classes to better fit the fact that PC's would stop advancing altogether. Essentially something like the "capstone" feats that E6 set up. And I wanted to address how the level cap affected the casting ability for rangers and paladins. The really important part, however, was to pick a point where the major spellcasting PC's would cease getting higher level spells. That was a HUGE factor in making E6 work was in limiting spells to 3rd level. I eventually decided that once any spellcasting class was able to cast a 5th level spell their spellcasting ability would cease. That still left a few key spells wanting/needing to be backdoored in - reincarnation for druids, stone to flesh, and restoration. I left open the possibility to have a few other SELECT high-level magics used on a highly limited basis, but for the most part that felt to me like it cut out the worst of the high-level spell imbalance.
One of the other easily overlooked factors is that in 1E/2E, demihumans were able to multiclass but at the cost of being level-limited. That didn't work well in 1E/2E because of the fact that unlike the days when 1E was new, nowadays most players would not retire a PC at "title/name level". That left a severe imbalance between human and demi-human PC's. But, with an xp cap of 700,001xp that stops even humans at or immediately after "title/name level" and actually makes the demi-human level limits FIT and generally make better sense.
Last thing to mention is that I wanted the 1E Players Handbook to otherwise still be accurate and reliable for the players use - they wouldn't need a massive set of house rules. With all the changes I made (including a much shorter, clearer initiative system) it was only about 20 pages of rules to make it work. I almost think that for 1E/2E it's just the IDEA that PC advancement can't continue forever, and that there is a significant limit placed on the power of spells that is the most important part of the concept. It really colors how you look at the game in the long-term, even if the level cap is more like 10th-12th than 6th.
It's at:
if you're interested. And obviously, anybody will want to tweak what THEY want to tweak about it. That link is simply where my thinking and my preferences led me.
Second edition does kind of have a natural cap around level 9 / 10 when players stop gaining HD etc, and really extending that to include spells and thaco wasn’t really game breaking. Although allowing the wizards and clerics to cast higher level spells as one off rituals with a bunch of time and money spent on research was a welcomed addition.
