[Aussie Mike]

One of the biggest things you will be fighting with it heat warping when welding the flanges. I did the final welding of the flanges with them clamped to a large piece of T section steel that kept the whole thing straight.

Welding them in position on the motor is a pain because you will be welding upside down plus you don't want to run the risk of anything disappearing down one of the exhaust ports. Fortunately you have those spare heads sitting there. Once you have the locations of all the pipes sorted on the motor tack everything in place and unbolt the flange and bolt it to the spare head. That way you can flip it around to get the easiest positioning for welding.

Don't worry too much about how the pipe fits into the port. Once welded flip it over and panel beat the pipe out to fit the inside of the opening in the flange. I did this using a steel drift that I ground the end with a dome shape. You can then just run a fusion weld around the port opening if you want. You won't need hardly any filler rod. That will unsure a good gas seal.

You probably still have a little warping after they are all finished it shouldn't be too much of an issue. The manifold bolts will pull a small warp out. Or if you want I can run the fly cutter over the flange on the mill.

Cheers