fuel tank location

I'd like to mount my Saito .56 on it's side using a commercial motor mount and a firewall. Then I thought "why not mount the tank right there in the engine compartment". Just make up a bracket so that the tank can sit beside the crankcase, in line with the carb, and surround the whole thing with a radial cowl. If I put the fuel feed in the rear right hand corner as usual would there be a problem? Would the head pressure changes be unusual compared to an inline tank location? Would the cg change be a problem with a forward mounded tank?

I'm not exactly sure that I understand your setup, but I believe you have the engine head on the outboard side, and plan to mount the tank on the inboard side, just inside the crankcase. If that is true you may have two problems.
1)The engine will richen up after takeoff. Assuming a uniflow tank, the uniflow vent is radially inside the spraybar position and you will develop a positive pressure head due to "centrifugal" force.
2)You may get an opposite effect of richening when the nose is picked up (and leaning with the nose down--not very comfortable on the downward legs of square maneuvers!), depending where exactly the uniflow vent is located. Since the carb on a 4 stroke is in the rear , you really would like the uniflow vent to be behind the spraybar.

Muffler pressure might minimize these effects.