I have made my own jig and used it several times very sucessfully.

You should use an LA head to build it on to get proper angles and heights.

First I bent a flat 1/4 inch thick flat stock steel to go around the deck surface around to the intake surface and drilled a hole through it a little bigger than the intake bolt hole and in the aproximate location of the bolt hole. Then I took a tube about 3 inches long with a 11/32 center hole and took a long bolt about 4 inches long and bolted it all to the LA head and welded the tube to the flat stock that was bent around the head and took it off and there was my jig. You have to make it on the center intake bolt holes on the LA head and when you drill the end bolt holes on the magnum head you have to use a 19/64 spacer on the head deck surface because the LA bolt holes are not on the same plane. I just use welding clamps to hold it on the magnum head and move it for each bolt hole.