It's been a long time, but IIRC I used an intake gasket as a pattern, cut the intake passages, and then sat the plate on a stock intake to pattern the perimeter and mounting holes. I did have access to a Bridgeport at the time, but I can't remember if I did it on the mill or 'freehand'. In retrospect, I kinda wish I'd gone a bit thicker with the plate sticking up farther at the top; that could have provided a tie-point for the motor mount arm on that side of the motor. What stopped me at the time was that I couldn't see a reliable way to duplicate that on the plugs- side of the engine. After the fact, I now think it would have been possible.
I used aluminum brazing rod to attach the 6061 tubes to the 6061 plate, trying to avoid warping the plate. My highly skilled/experienced neighbor suggested using a steel plate and stainless tubing so it could be TIG'd, contending that it would make very little difference in weight. In retrospect, he was probably right, and steel would have made using the intake flange as a motor mount very simple compared to using aluminum, with the restrictions/uncertainty of structural welding on aluminum.
FWIW...