A kindly person from the Baccus forums lent me a look at the Bull Run scenario from the Eastern Battles scenario book, many thanks! It's a good basis from which to start developing my own version of the scenario. The "official" scenario is designed for the 150 scale which means there are 1/4 more stands in the brigades and the groundscale is 1/4 larger than what I'm going to use. Conversions should be easy enough. Looking at the map I see the designers have bent the real locations of things a bit to fit the scenario. I suppose it's ok if it fits the table better while still keeping to history. Also, I see there are some details of the battlefield which are different with different maps I see. For example, the wikipedia map I looked at doesn't have woods on the southern half of Henry House Hill, whereas many other sources do. This is an important detail I'll have to look into. I will be double checking the terrain and OOB from any sources I can lay my hands on. One promising source I found was this blog with some digitized old records of the war. It's the first source of manpower information of the brigades I've come across.
A bit of a lazy man's source, I'm also in possession of a civil war PC game from a few years back, Take Command: Second Manassas. It's a sequel to a game which had the battle of Bull Run, but which is no longer available. The modding community for TC2M has been active however, and has ported the Bull Run scenarios of the previous game into the second. I can take a look at the well researched information on regimental level strengths of the combatants in these scenarios as well as see a 3D implementation of the battlefield, which'll give me a good idea on what the terrain on the tabletop version of the battle should look like. TC2M is the game which got me interested in the American Civil War for wargaming in the first place. It's still a good game (verified yesterday) and you can get it for dirt cheap on Steam. Also, take a look at the successor for the game, Scourge of war: Gettysburg, which improves on the recipe by adding multiplayer. And no, I'm not getting paid for this advertising.
I'll end with a holiday snapshot from a Pickett's charge scenario from SOWGB.