Header flange to round pipe interface

The slant 6 header flanges have square holes in them to match the square holes in the head (intake and exhaust ports.)

To build a set of headers from these flanges, what's the best method to change the shape of the primary 1 5/8" header pipes to square, to match the holes in the flanges? Does anybody sell short pipes (square on one end) that could be used for this?

Any information will be appreciated...