Reliant carb turn round?

No, that's fine Rod. Just make sure you can route the throttle & choke cables O.K
Also check it doesn't protrude beyond the line of your arms & legs when sat on the trike -during the MSVA/BIVA trike registration test, they check for pedestrian unfriendly sticky-outy bits by running a large foam roller down the side of the vehicle. It's done with the rider aboard though, so remember to stick your elbows out!
Reliant mounted the carb facing inward over the engine to pick up warm air for cold weather running, but Flap's & Muffin's trikes both run them facing outwards. Taz's & my own builds have the carb turned through 90 degrees to face forward. If you look at the mount on the carb manifold, it's diamond shaped. The older 750cc Reliant motors used Zenith downdraft carbs. Same spacing on the mounting studs but 90 degrees around from the SU carbs. So you could fit an early Zenith manifold to turn your SU through 90 degrees -or just redrill the later ones like we have.
Have a look at Taz's trike project thread, (stickied at the top of the General Discussion page), & you'll see the 6" Harley bellmouth she's using, fitted to the carb with an adaptor plate made by Doc, ("Twisted Patience").