The Bombard in EpicA is really rather an anomaly. The Epic Space Marine 1st and 2nd edition Bombard was an extremely long range, powerful, slow firing artillery piece built onto a Leman Russ chassis, with no other guns:
Unlike the metal Epic 40k one the current 40k FW Bombard matches the original one:
If we to do that Bombard properly in EpicA it'd have a 10cm move to match it's extremely slow speed in the background/rules (best with 'May not Garrison' in it's notes though), either a 5+ or 6+ save (it's open topped and there's zero room for the gun crew inside it, however it's built onto a tougher Russ chassis to start with), no heavy bolter and 120cm range on it's gun, probably for 300 points or so.
The last IG Codex introduced the Colossus Siege Tank, which is similar and closer to what we have in EpicA, being mounted on a Chimera chasis and having a heavy bolter. As with the Bombard it's gun is equivalent in range to a Basilisk though, there isn't a short ranged siege artiillery gun like the EpicA one in 40k at all. I'm not sure what the best option would be - I'd like to adopt proper stats for the Bombard as above, but that might be too big a change for some people? In which just case re-naming the epic Bombard a Colossus could be an easier option to fit better with 40k (as is a 40k player coming to epic is going to find the epic Bombard way off what it should be).
Griffon going up to 5+ save in the SL list is appropriate and could make it used more commonly.
I think the Manticore should be left as is. Perhaps there could be justification for Basilisks going to 225, I don't know either way, personally I always choose Manticores over them.