If you want a trick N/A motor, you can replace your carbs with EFI throttle bodys bolted to the sidedraft manifold. A company in Australia makes them, used to be called EFI Hardware I think, but it would all add up to a bit of money. And not so suited to turbo.

The 18R-G head has no provision for injectors, like what the 4A-GE has. So if you can't find a 18R-GEU manifold, you are going to just find any old EFI manifold that faces roughly the right dirrection, (idealy with provision for injectors) and make it fit. Start with a alloy plate and make the holes to fit the head. Then chop the old flange plate off the new intake, and use a aluminium welder...