Main vent location

Hello,
Ive got a job where there are no main vents on the 2 mains. The long main is 3 inch and is approx 65 feet long and the short main is 2 inch and about 35 feet long. The long main had no provision for a main vent. Although not the best location, I installed a tee at the end of a 6 foot dry return at the exit of a crawlspace before it dropped down to a wet return. I will pipe the vents up and over to prevent condensate hitting the vents. (see attached photo, please ignore the hoffman vent the 2 Big Mouths for that are on order). The short main I am not so sure about. It ends in the back of a different crawspace and there is a 2x3/4 tee before it bends back into a dry return that runs all the way to the boiler. The issue is the tee is turned horizonal. In such a tough spot trying to repipe it so its facing up is near impossible. Is it ok to come out of the tee horizonally with a nipple then 90 up and attach the Big Mouth? If that isnt ideal, I could always attatch 2 Big Mouths at the end of the dry return back by by the boiler (about 70 feet from beginning of main to end of dry return at boiler). Just wondering what the best option is. Thanks