You've had a lot of good advice here but it is a bit all over the place.
Dave has a good point, you don't say what you are shooting the ammunition in but I'm assuming its a military spec rifle, in that we are not talking match grade tolerances here and a lot of the arcane reloading practices of the benchrest mob can have little noticible effect on a military rifle targets when you are dealing with variable barrel conditions and the inherent weakness in military rifles for shooting good groups and testing the accuracy of rifles and ammunition, the sights. Your rifle and ammunition may be capable of 2" groups but your sights may mean that you can only shoot 4" groups.
With all the experimentation that you've done so far you have yet to try the one thing that will make a noticable difference to your groups, finding the best powder and velocity, for your rifle and ammunition combination.
You don't say what your range goal is in developing the load and that would be helpful.
If you are only wanting good accuracy out to 2-300 yards then a faster powder and cast boolits will be the way to go with ease, if you aspire to longer distances then unless you cast your own boolits it is probably easier to go to condoms (jacketed).
By defining purpose for your loads you will then be able to select a suitable burning rate of powder.
You have chosen Alliant 10x, not a powder I have much experience with but it is in the VV N130 N133 burning rate, the charge you have chosen is the one recommended by Alliant for the .303" British, unfortunately Alliant don't give maximum and minimum charges, however a run on the data available through Quickload indicates 42172 psi for a velocity of 2360 fps. SAAMI max pressure for the .303" British is 49000 psi, 35 gns gives you 43702 psi. Quickload won't give exact results but it is a good indicator of where you loads are, personally I would consider the Alliant load as a maximum, the velocity is there and you don't seem to have noticed any pressure signs, so why go any higher.
If we take the 34.5 gns as a max then I would suggest 30 gns as a starting load which should give 2000+ fps with 30000 psi.
Load 5 rounds each in .5 gn increments starting at 30 and working up to 34.5, 50 rounds in total. Neck size the cases using the Lee collet die which is one of Lee's finer efforts. As the cases are fireformed to the rifle then neck or in this case collet sizing the rounds helps keep the bullet centred with the bore. The bullet is the most important part of the load, its the projectile that travels from rifle to target and any imperfections in the bullet will lead to poor accuracy so we want to get that bullet/boolit entering the rifling as concentic as possible so that there isn't any deformation.
Keep the OAL the same for all cartridges I would go with the recommended OAL of around 3.070" so that it fits the magazine, ignore the cannelure position on the bullet if there is one.
Shoot your five shot groups letting the barrel cool down 'tween groups, one set of five shots will be better than the rest that is your load with that combination of components. You can try and refine that a wee bit if you want by doing a further series of 5 rounds at .2 gn increments 'tween the load before and after your best load in the series, you should be looking at a 100yrd 4" group being your max and with luck it should be around the 2" mark.
If you don't get groups in this range then I would look for another powder and go for a slower burning rate starting at VV N140, I've also had good results with N150 and my current favourite AA 4350 all a fair bit slower than your current powder.
The next step for geting better groups would be to change your bullet to the Sierra 174gn .311" Matchking as I've already said with the bullet being the most important component it pays to get the best you can and these MKs are proven performers. You may not notice a difference at 50 yrds but you will beyond 2-300. If you are only shooting at 50 then as has been previously said cast boolits are the way to go.
Finally on OAL you really need to take a chamber cast of your rifle so that you can se what your throat, lead and chamber looks like and mikes out at. Depending on the barrel and the amount of throat wear you might never be able to seat the bullet out enough to get near the lands, so do a chamber cast first so that you know what you are working with and that will inform how you make your ammunition.
If you can seat to the lands then start fiddling with OAL once you done the other development, work first with the elements that will make the biggest difference, powder - burning rate and velocity, and bullet choice, they can bring a 4" down to a 2" group, bullet seating can help improve groups but not to that magnatude.
As you have already alluded to only change one thing at a time, set yourself parameters - distance to be shot at, maximum acceptable group size, cost etc and then work methodically towards that.
But then what would I know.
