More HP means burning more fuel/air or making more efficient use of what you're already burning. Then ridding the engine of the spent gasses.
A few random thoughts:
The rear gears will definitely give you more "seat of the pants" performance. But the down side is, unless you have an OD trans, highway cruising gets old, fast.
The exhaust is a good idea. You should also be running long tube headers along w/a true dual exhaust w/an "X" or "H" pipe connecting the two sides, placed aft of the collector, fore of the mufflers.
Using a small starter can help matters when using headers, but isn't necessary. I wouldn't use one unless I had to buy a starter anyway- at least not at this stage.
An intake like the Edelbrock RPM would be a good improvement, along w/the carb of your choice. The Q-jet is good, but most guys end up w/a Holley. Around 650 cfm-750 cfm will work, I'd suggest a 3310 vacuum secondary 750 Holley as a good carb if you're not up to working w/the Q-jet.
An MSD ignition box can be added to an HEI distributor. Unless your distributor is worn out, just swapping it for an MSD distributor won't add power, but the adjustability it offers, could. This can also be done to the OEM distributor, so it's not easy to say yes or no for sure w/o knowing more about what you now have. But setting up a proper advance curve is key to making power and avoiding detonation. The exact specs depends on things like the compression ratio, cam, what heads are on the engine, the fuel used, that sort of thing.
An MSD box like the 6A/6AL might make a small difference in power, but at the RPM levels that you are likely to see w/your SBC 400, I wouldn't make it my first purchase.
A good fuel pump is a good thing to have, as is a regulator or to at least check to see fuel pressure is at 5psi.
If you have a stock cam, this is another thing worth looking into.