The 25mm pipe just gently curve it in the direction you want and after the bend I cut it, bought a length of 45mm exhaust pipe, capped one end and fitted a 25mm internal dia bit of pipe (actually was a bit of an old floor jack handle) but you may be able to get the exhaust shop to fabricate it easy enough, just stuck the 25mm into it with a bit of exhaust bog, hung the pipe from the cross members/floor with straps and it comes out just behind my wheels.........make sure it is slightly lower at the end so no juice pools in pipe.............
I used the larger pipe to get better exhaust flow as I think I cooked my first heater by having too long a length of 25mm pipe, this larger dia works well and is about 1.5m or more long, with a ball of stainless steel wool shoved up it to stop crawlies and sound damping.
My heater is near the door so didn't want it blowing near there at all......
I also did a similar thing with my air intake, the 25mm pipe is just for the first bend and then it goes into a length of 40mm PVC pipe with the filter screen jerry rigged to the end of it, it is just about 1.5m long............
I put a stone guard in front of my heater, made up of a bit of 200mm C channel with one side cut off, certainly stopped a few errant rocks going by the knicks in it.