I have a grizzly big boar that I am going to have rethreaded to take commonly found brakes(and I just sent off a form 1 for a suppressor in .50 cal). It won't cost any extra to cut whatever length I want off of it while doing so. It is a 36" now. I don't see myself shooting beyond 1000~1200 yards(mostly under 800 yards) with this rifle, but I want to make sure it has true 50 thump at those ranges(what's the point in shooting APIT's if you lack the velocity to make it "AP"). It seems like 29" is one of the more common found lengths.So, how short dare I go? And, could reloading help make up for the difference, i.e. different faster burning powder. Or, at the very least, if I will have 2X inches of barrel and a huge muzzle flash,could I reduce my loadings to cut out the muzzle flash? Maybe a little cheaper to load? Thanks in advance.

A faster burning powder will increase pressure and reduce velocity.  Keep that barrel as long as you can.  My 29" barrel on my AR-50 pushes 650 grain API's at 2750 using 212 grains of WC867.  I tried using Reloader 50 to bring up my velocity and it did.  Using the slower Reloader 50, I was able to get the 650 API's up to 2,900 fps but accuracy suffered.

Well 29 would be a good reduction in length and offset almost half the length of the can.

I've used barrels as short as 16" on a 50BMG State Arms rifle - did not measure FPS. Used it for shooting off-hand. Barrel was heavily fluted also to reduce weight forward.
My AR-50 currently uses a 33" Feddersen barrel with a 0.554" neck tight chambering.
Prior to that it was 35" long and used for velocity testing - Using Barnes 800 grain Long Range Solids and coated bore/bullet technology, I was able to push them almost 3200 fps. I will not tell the loading as it was heavily compressed and without the coating technology use - it would be a grenade in your face. Barnes 800 LRS are not accurate at all at long distance and I will not ever use them again. Cutting Edge Bullets or Lehigh Bullets are my current only choices for accuracy.
My standard match loading for FCSA Matches is 244 grains of either 20N29 or Reloder-50 with 510-170 808 Lehigh's at roughly 2800 fps. Powder choice is dependent on temperature and elevation in conjunction with barometric pressure.
Lowest Standard Deviation is a requirement for accuracy match loadings used at 1000 yards or longer.

YMMV.