yeah I've done it, cant find the bloody camera though..
The "log" of the standard manifold is too small, so you should fix that while you're at it.
There's a photo of someone else's attempt (much prettier than mine) in the latest 4wd custom guide issue number 34 page 12. I do have a criticism of their execution though, it restricts access to the injection pump for repair or adjustment too much.
I milled the top of the manifold off apart from the last 60mm or so at both ends.
then made up a box on top from 3mm ali plate. just over three inches high, front and back of the box slope down at 45 degrees to meet the top of the original manifold.
3 inch mandrel bent pipe comes out the side of the box curving forward and downward slightly.
I'm running 20 psi through it. Making reasonably good power but I haven't seen many others run three inch pipes, most seem to go with 2.5 inch.
Make sure you get someone who knows what they are doing with welding it. and clean the manifold so its completely spotless. make sure you use clean tools to cut and shape the ali because metal contamination makes welding a bitch.
when welding bolt the manifold to a spare head or large lump of steel. It WILL warp, its just a question of how much.