To make the header itself fit, you have to cut off the flange for the downpipe, turn it 180 degrees (to clear the engine block) with a slight angle change and weld it back on. There is not enough room for this flange if you do not turn it around 180degrees and tilt it a bit.
Lay down a B20A exhaust gasket on top of the H22A header flange. Use a permanent marker to mark up the stud holes and exhaust ports according to the B20A gasket and grind/drill out the new stud holes. The two center exhaust ports will need some porting to match as you can see. I think the 2 center ports are offset by about 2mm between H22A and B20A so it's no big deal to portmatch. One side of the wall you will need to grind off some metal. Try to make it a smooth transition and do not grind any deeper into the runner than right before the start of the weld that attaches the header flange to the runners (to avoid grinding all the way through). The other side of the wall where the gasket is covering part of the exhaust ports I chose to fill up with a welder and then I used a grinder to even out the shape.
The downpipe needs to get its angle changed by quite a lot otherwise it will point straight into the oilpan. So you need to cut the downpipe like shown in the picture and of course also rotate the flange 180degrees and then weld it back together. The two small pieces you can throw away or weld yourself a new tailpipe or whatever you feel like doing with them…
From this part of the header you may choose to make your header different than I did. I chose to replace the DC sports 2” collector pipe with a 2½” pipe and flexpipe. To match the 2½” pipe with the downpipe you just need to crush the 2½” pipe into the same shape as the downpipe. Then you cut the downpipe at the collector joint where the circumference is the same as your crushed 2½” pipe (use a tape measure).
If you want your downpipe to attach to the exhaust bracket on the back of the engine you have to cut off the bracket from the DC sports downpipe and weld it onto your new downpipe. The easiest way to do this is to bolt up the header and downpipe. Bolt the bracket to the exhaust rear engine bracket, align the downpipe with the bracket (not much alignment possible but you can tweak it a bit) and tack weld it on. You can take the downpipe back off to get more room to complete the welds. Also remember to weld an O2 sensor bung back in to your new downpipe.
Here are some pictures for you of the complete exhaust system I made at the same time.