Too much powder? Beginner needs some help!!

[69roadrunner]

I have a 38-378 Accuramark and I found the best powder for my gun to be Accuate 8700. My gun likes 121 grains behind a 180 grain nozler partition. I tried RE-25 and found it too hot! I recently discovered my throat is eroded away and needs to be re-worked. What are the opinions about a tight throat vs. freebore? When the gun was new I could safely load to a length of 3.900, but since they wouldn't fit in the mag. I had to reduce my length to fit.