Please note that with H110/296 it is NOT recommended to reduce powder charges below 3%! Those powders require a (near) full case over a mag primer to effectively ignite.
2400 is a more forgiving powder from that perspective. IF you absolutely want to push the magnum limits, H110/296 & Lil Gun are the powders to beat. If you want a good, solid, high power load that is not an all out "Max" load, 2400 is likely the powder of choice. I personally prefer 2400 for the added flexibility of loading.
Your comment on bullet size causing excess pressure indicators: Yes, it can. But let's consider some things: Lead bullets are NOT sized the same as jacketed bullets. A jacket .357 bullet mikes out at .357 (I think. Might be 358. Play along here.) whereas a lead bullet will (virtually always) size out .001 larger (or in Cowby Action, often .001 - .003 oversized). So a lead bullet will be .358/.359 in size. This is normal that lead is sized slightly larger than a jacketed bullet - it's intentional.
Gotta make sure you are comparing apples to apples here: in this case, jacketed to jacketed bullets and lead to lead when it comes to sizing.
So, let's say you used a .358 sized Jacketed bullet instead of a .357 jacketed bullet (do they even make one? I don't know for sure here.) then yes, the added case tension on the oversized jacketed bullet can increase case pressures. If you choose to use an oversized bullet, start your load development again with miminum recommended charges to be safe and check for pressure signs.
Better safe than sorry.
And as Wilson said, don't worry about not filling up the whole case. Some powders (such as Titegroup) are designed to work with minimal charges and off kilter powder positions in case. But said TG loads are also extremely easy to double charge.
Double and triple charges = EXTREMELY BAD JUJU!
Hope that helps.
"Remember, Eagles may soar but Weasels don't get sucked into jet engines!"