I will start by saying that I have not completed this project yet!

Take a look at the throttle bodies from a 1000cc GSXR. The bore is 40mm which is roughly the same as the venturi insert on a 45 weber. The throttle body has the boss for the injector built into the body. The bores are slightly closer together than the barrels of a side draft weber. (thus requiring a custom intake manifold) There is a throttle position sensor attached to the throttle bodies. They are relatively cheap on e-bay (at least here in the States, I bought a set up for $65ish, shipped). It looks like making an intake manifold will not be too hard. (as your father was going to weld the bungs for the injectors into your current manifold he can probably do this type of work, too) I am actually fabricating one for an SR type engine. I think that there are examples on the picture page of some one doing this so search around a bit and it may give you an idea.

As an example view this handy photo;
http://datsun1200.com/modules/myalbum/photo.php?lid=14254