Curious if its possible or a good idea to cut the flange off a header and welding on a new flange.

For example:

Cutting off D-series header flange and welding on a B-series flange.

 

so long as the head port spacing is the same, and your loss of length in the header tubes isnt too extensive to cause clearance issues. just make sure your cut across all 4 tubes is as straight and flat as possible, then clean, bevel, weld. and make sure you use a spare head to fasten the flange to so it doesnt warp

 

You could grind the weld out with a die grinder and not lose any primary length.

 

B series center to center port space is 3.54". D series is 3.30". Would likely work ok, just cut off the flange with a vertical band saw and use a metal messaging device (sledge hammer) to fit.

 

As stated above it isnt ideal but you could make it work. If you have the skills and equipment to do this, you probably are capable of removing the flange and first few inches of the primaries in order to redo it to fit the slightly different layout and not have to bend things.

 

Possible? Yes. Worth it? Not in my opinion.