I have a Cox .049 postage stamp backed 290 donor engine from crashed and totalled late 1960's RTF. (This was a predecessor to your horseshoe back engine.) I used a Carl Goldberg nylon accessory engine mount, which provided mounting lugs. On the Goldberg Little Toot profile fuselage biplane, I cut a portion out of the balsa wedge behind the engine firewall, so I could get the tank closer to the engine. Then I glued the tank to the fuselage.
As greggles47 stated, I'd place the fuel tank vertical centerline in line with the venturi air intake centerline and as close to the engine as practical.
Tank appears to be wide enough extending pickup past the right side of the engine. To allow for ready attachment of the fuel line, you could probably drill a hole through the firewall to allow the tank nipple extend past the firewall, then glue tank up against the firewall. That will require trimming of the balsa cross brace supporting the firewall that is fitted in that slot in the fuselage. (I think that is what that large slot up front is.) This will make for a very short fuel line run. Plus, the tank will help support the firewall where brace was removed, or you could put sheet balsa or plywood triangle pieces above and below tank for additional support.
If you put the tank a little ways back, (say two 3/16" to 1/4" balsa blocks laminated together in front of tank above and below brace wedged between firewall and tank), trim brace to clear tank) then make the hole in the firewall larger so fuel line can pass through it could be another way.
I've attached a few images for examples. The 18" wingspan Scientific Little Devil shows a beam mounted engine and fuel tank installation. It and the P-82 are full fuselage, but I think you'll get the idea. Also show the Keith Laumer profile Gee Bee Z. Installation is for a beam mount engine, but he put the tank up close to the engine.
Reason for wanting to keep the fuel line as short as possible is these engines with their diminutive fuel flow don't have strong suction. It's why Leroy Cox and the Herkimers (OK Cub) came up with integral tank engines. It solved the problem of feeding these little engines. If you look at any of the Cox or Wen Mac RTF's with tankless engines, you'll notice the fuel tank compartment molded into the fuselage is close to the engine.
You probably already know this, but when you test run your engine, due to the tank fuel pickup location, will only be able to empty half the tank unless you hold the plane with left wing tip up so tank pickup is lowered. Once in flight, centrifugal force will cause tank to properly empty.
These are just a few thoughts. Good luck on your build.