Thanks for the link, Rastamann. Interesting reading. Not quite what I was thinking, though, so I decided to continue with my own Legion army list and see how that goes.

I see that they struggled with how to make Marine formations larger without making them unbreakable (or at least incredibly difficult to break). My solution has been to remove ATSKNF for Heresy-era Marines (the pretext being that there is no Cult of the Emperor yet, and as such the Marines are not yet zealots), which enables formations to be twice the size (or near to it in some cases) for only a tad better breakability. In the same vein, I have also removed access to Chaplains, but also to Librarians as I understand Marine psychic powers were in their infancy in M.31. (Happy to be corrected by those more current with the fluff, of course.)

I also added a rule that Rhino transports are not counted when calculating the units-to-blast markers ratio to identify broken formations. This helps balance the breakability of the larger Marine formations.
The result is larger and tougher but less flexible formations that have, I think, the right feel for a Space Marines Legion.
My mathhammering seems to bear it out...but I've been wrong before. Happy to take any and all feedback.
I have posted one of my Epic:Vassal scenarios' text and the Legion army list used to create the forces (Sons of Horus vs Imperial Fists) below. Let me know your thoughts. And if you'd like the actual Vassal save file so you can see the Deployment Area window, just PM me your email address.

THE HORUS HERESY: A FEINT BETWEEN FRIENDS
The Horus Heresy has begun and it is the early stages of the War for Terra. The
Warmaster has sent nearly a full company of his Sons of Horus Legion toward the
Imperial capital in a probing feint intended to draw out the Emperor's defenders
and enable him to better determine the level of opposition.
Horus himself leads the attack, not knowing his brother primarch, Rogal Dorn, will
marshal the defense personally, albeit with an under-strength Imperial Fists
company.
FORCES
See the Deployment Area window. Force strengths are
Imperial Fists: 4,000pts
Sons of Horus: 6,425pts
OBJECTIVES
Horus must secure the following objectives:
- the Tank Factory
- the Materiel Store
- the Administratum building, or
- the Censorium building
WINNING
Horus can claim victory from the End Phase of Turn 4. If Horus has not claimed
victory by the End Phase of Turn 6, he has been defeated and Rogal Dorn claims victory.
NOTES
- This is not a balanced scenario.
- Walls are Armour Save 4+ (RA) DC3 war engines, and they provide a 3+ cover
save to any infantry unit placed atop them. Walls are almost as tall as a Warlord titan,
and so block LOS completely for all units except flyers that are within 15cm.
- Towers are Armour Save 4+ (RA) DC4 war engines, and they provide a 3+ cover
save to any infantry unit placed atop them. Towers are too tall for any unit to see over
them, and so they block LOS completely.
- Towers are armed with Defense Lasers. 90cm. MW2+ (TK, D3). They have no FF or
CC value, however, and so cannot "fight back" if engaged in an assault. Note that they
_can_ benefit from Supporting Fire, however, if so engaged.
- The bridge is an Armour Save 5+ (RA) DC2 war engine, but it provides negative cover
(+1 to hit!) for any units crossing it when fired upon. (Defenders should consider
overwatching the bridge!)
- All other buildings provide cover and a 4+ cover save, but are indestructible for the purposes of
this scenario.
=============
PRE-HERESY SPACE MARINE LEGION ARMY LIST USED FOR THIS SCENARIO
NOTES
*Pre-heresy, so Space Marines are not yet religeous zealots, and therefore do not yet enjoy the And They Shall Know No Fear special rule.
*Rhino transports are not counted when calculating the units-to-blast markers ratio to identify broken formations.
SPACE MARINE LEGION HQ
Mournival HQ (1 Primarch [Supreme Commander] + 5 Captains + 3 Land Raider transports)
600pts (375pts + 225pts)
- A Mournival HQ can be taken only for battles greater than 3,000pts.
SPACE MARINE COMPANY BUILDER
0-2 Terminator Platoons (6 Terminator)
350pts
Upgrades
- Commander
- Add 6 Land Raider transports @ +450pts
=OR=
Add Droppod transports @ +40pts
0-7 Tactical Platoons (12 Tactical + 6 Rhinos transports)
675pts (transports are free)
Upgrades:
- Commander
- Replace all transports with Droppod transports @ +40pts
- Replace all transports with 2 Thunderhawk Gunships @ +300pts
- 0-4 Dreadnoughts @ +25pts each
0-4 Devastator Platoons (6 Devastator + 3 Rhino transports)
300pts (transport are free)
Upgrades:
- Commander
- Replace all transports with 1 Thunderhawk Gunship @ +150pts
- 0-2 Dreadnoughts @ +25pts each
0-1 Dreadnought Platoons (8 Dreadnoughts)
300pts
Upgrades:
- Dreadnought types/classes can be taken interchangeably
- Add 2 Thunderhawk Gunships @ +300pts
=OR=
Add Droppod transports @ +40pts
0-2 Heavy Armour Squadrons (10 Land Raiders)
650pst *equivalent to 10 Chaos BL LRs
Upgrades:
- Commander
0-2 Medium Armour Squadrons (10 Predators--use Chaos Predator stat line)
500pts *equivalent to 10 Chaos BL Predators
Upgrades:
- Commander
Upgrades
Commander: Veteran Sergeant (see Captain rules) @ +25pts