You need to look at the working area for Z height between the 440 and 770 and see if you can get the boring bar in the 440 and clear your workpiece. The 770 is a lot more machine (in comparison). So don't nickle and dime yourself out of what you need for the job. If it's marginal, don't buy marginal.
1.5" of flute stickout on a standard end mill on any Tormach is a lot and might give you major problems in stainless. You may want to look at insert cutters to work your way down the pipe or maybe a high feed low engagement cutter like or similar to the Tormach Dijet. It won't be big cuts but it will be reliable fast low z depth passes with small hp requirements and no big rigidity problems.