Newbie needs some imput oversize bottom bracket

I am building a GT Kustom Kruser Double Down. I loved the look of this bike and knew it would be a challenging build. I have run into a big problem right out of the gate. The bottom bracket is oversize at 180mm. I purchased a sbk shift kit and the bottom bracket and crank set are not compatible with the oversize setup that came with the bike I am currently looking at cutting back the bottom bracket housing I have looked inside and there seems to be enough thread cut into the housing to make the a option.

I need some input on cutting aluminum tubing and not screwing up the threads as well as keeping the cut true and straight.

Tell me what you think

Thanks