In rebuilding a trike for a friend and he wants to alter the fuel tank to a motorcycle type instead of a car type. Problem is the reliant engine has a water jacket bolted to the inlet Manifold that the carb bolts to at an angle causing the carb to be too high so it would foul the tank

What I need to know is does the carb need to sit at an angle or can I mount it verticle as my intentions are to fabricate a manifold to allow the carb to be mounted lower ie level with the inlet on the head thus allowing me to fit a bike tank.

I would think as long as the float bowl is fairly level you wont have any problem
On older SUs you could move them by slackening off the bolt that secures them
That said my bike carbs are far less of an angle than they were on the bike and theyre fine
Theyre similar in operation to SUs

What model of SU? standard H and HS types are sensitive to angle Land-Rover used Zenith-Stromberg CD carbs on the V8 engine because of this.
With the HS series it might be possible to alter the angle of the float bowl.
The more upmarket models with an annular float bowl concehtric with the jet such as the HIF and HIF should be less sensitive to mounting angle.

su carbs, well the hs2, 4, 6 &8, are designed to be vertical, with the black cap on top of the plunger assembly at the top. they can be mounted up to about 40 degrees, but the float chambers should be set back to vertical.
you can also get 'stubby' su carbs, (late dolomite if I remember correctly)

On the HS carbs the float bowl is bolted to the body with a single bolt, with a thick washer with a tab on each side which sets the angle of the bowl relative to the carb body. If you cut the tab off on the bowl side you can mount it up and set the bowl to horizontal, nip the mounting bolt up and set you float and needle levels....

Can you turn the SU carb through 180 degrees to get the carb to clear the tank?
My Reliant engined Austin Seven Special had the carb mounted this way round (carb intake facing away from the engine).