i built an aluminum adapter after i snagged an early steel one.
that adapter also came with the crank flange adapter that adapts the crank to the flex plate.
it was fairly easy to do.
i could dig it out and get the dimensions for everything if you wanted to try fabbing one.
like was said, the crank needs tapping, but the holes are 7/16", which is the correct size needed for the 1/2-20 flex plate bolts.
however, it would be wise to use a tap guide block [which is easy to fab up] when you are tapping so you keep the tap parallel with the crank holes.
another thing to keep in mind. i would also gut the case of the transmission you will be using, then dial in the pump locating bore in both planes with offset dowels.
this would make sure there are no tolerance stackups that will cause problems down the road.
edit : i forgot to add, this adapter is, i believe, a transdapt unit, which is kind of a 2/3rds. meaning it is only 2/3rds of a circle, while the hotheads piece is a full circle.
it also requires a B&M flexplate which i believe is different than the 8 bolt flexplate used on 426 and 440 engines using an 8 bolt crank flange.
i have the required flexplate, but it is on the shelf with the adapter.