I installed a Webasto5000 two years ago on my Bav38 (2003) using all the holes already cut by Bavaria.
I cut only holes for vents. I put them in the two aft cabins, toilet, dinette and forward cabin.
To avoid 90° bends just out of the burner, i fixed it on the wood tablet under the stern step, behind the rudder
I used a 9 cm Webasto duct as main duct and 6 cm for branches, inside a neoprene tube.
If you like i can share a document ( with italian notes) to calculate diameter and air flow from WEbasto installation manual, the items used with part number and some pictures.
regards
Maurizio