I fitted an off the shelf Vetus 80W holding tank to my 36 Classic from 2001, holding 80Ltrs. It was as other folk have stated quite straight forward (more by luck really than planning). So my Classic has 2 aft double cabins. I decided the simplest place for it would be the Stb Aft cabin large locker. The Vetus is shaped like a large pack of cigarettes in its proportions and so fitted very nicely in the locker up against the bulkhead between the locker and heads. This meant short pipe runs.
So I cut a rectangular hole in the locker floor about 2 ins shorter each side than the tank so the tank sits mainly on the locker floor. In order to support it more and also raise the height of the tank a bit (to make it easier to house the outlet seacock directly beneath it) I screwed a 1 inch thick piece of teak batten around the floor hole, upon which the tank sits. Then cut as neat a hole as possible in the locker base front panel in order to fit an outlet seacock. I retained the cutout and glued wood molding to it and stained and varnished said molding to make the cutout match the locker doors and used it with easy push on and pull off clips to make a 'doorway' to the outlet seacock. (I'm not on the boat right now but will post photos when next out in September).
I then Cut a hole in the hull large enough for 1.5 inch DZR seacock. Glued marine ply around the inside of the hole in order to add supporting strength. BTW had to cut hole in inner wooden liner to get to the hull, but this is only for insulation and cosmetic reasons (I guess as it is pretty thin ply), rather than structural. Then fitted seacock.
Then above the tank I cut hole through side deck and inner wooden liner to fit deck pump out plate. Bedded pump out plate and connected it via 1.5 inch sanitary pipe to the pump out spout at the top of the tank.
Then cut .75 inch hole in the topside about 1.5 inches below toe rail for a breather tube, and connected it via .75 inch pipe to the breather outlet spout on top of the tank.
Now the tricky bit was fastening the tank to the bulkhead. It is fitted with 4 flanges to take bolts to secure it. I marked the position of where these flanges were up against the bulkhead, then moved the tank out of the way. I drilled through the bulkhead from the locker into the heads. The innermost two holes came out nicely above the heads bowl and the outermost were inside the locker alongside the toilet bowl. Fastened all 4 with SS nylock nuts and bolts. Yours will be easier as you have no locker in the heads compartment where the outermost holes will emerge.
Next I cut the outlet pipe from the toilet bowl and inserted a 'Y' diverter valve to the lower end. This allows the toilet to either discharge into the tank or directly out to sea. I added 1.5 inch pipe to connect the diverter to the inlet spout of the holding tank. This had to run through a 1.5 inch hole in the heads/cabin bulkhead. I fastened the diverter valve to the bulkhead.
I double clipped all pipe connections, and ran short length of 1.5 inch pipe from the bottom of the tank to the new seacock. Finally I fitted a level sensor about 9/10 of the way up the tank, by glueing it to the outside of the tank. I took the power feed for this from the cabling running to the cabin side light. Not ideal as it only works when the cabin lights fuse is on, but I didn't want to run a long length of cable just for this.