While straight up is best, a 45 degree incline might work for a steam stack, but you may want to increase the diameter, enlarge it 20% or add an inducer, whichever costs less. Steam stacks can be light gauge, If you get it set up before the season, soon enough to test it, I suggest running a test just boiling potable water without enlarging it to test.
On my old 3x8 I had a full hood but with a devider between the 3x3 syrup pan and the 3x5 flues pan. I had 2 steam stacks, both 15". The stack on the syrup pan had 2 45 ells, to side step a joist, the other had a preheater in the hood and a lockable damper to hold steam in to just before steam started leaking out at seams where the hood sat on the evaporator, or thru the 4 doors in the hood. That damper was locked at about 45 degrees closed, but that stack went straight up, no offset. It worked fine, so much so that I often thought the stacks would have been good at 12" each. I'll never know, I sold that evaporator complete as I downsized. I'm now setting up a 2x6 oil fired for the 2026 season. Having gone from over 1300 taps down to under 500.