To physically fit, the single-point distributor will need to be from an Eldorado or at least you should use the base from an Eldorado or you can use the V700/Ambassador distributor and a reducing ring to downsize the hole in the engine case (this is what Guzzi did on the very last of the Ambassadors). The reason I say this is the V700/Ambassador distributor base has a smaller diameter "step" on it than the Eldorado one or the dual-point timer.
The drive gears are different - you'll need to swap the gear from the dual-point timer to the single-point distributor.
You may need to recurve the advance of the single-point distributor to suit the needs of the T3 engine. This could be as simple as a different set of springs.