Don't do it!
Seriously though buy a front clip with good A pillars all the way up.Find out where the rot stops on your original A pillars and leave as much original as you can.Then a few inches above this take off the front panel skin from the a-pillar like this
As you can see it is spot welded on the inner windscreen lip and also it curves round the back and spot welds on,then cut a slice top and bottom and gently pick it off,you'll want to reuse this so work out where to cut carefully.
You'll then notice that the A pillar is actually two half box sections spot welded together.So ideally you want to cut each section of the a-pillar at different heights to give a staggered joint for more strength like this
Once you've joined the a pillars back together treat them with an anti rust weld through paint.The you'll need to cut your outer front panel section to size and join it back on.
Brace the a-pilar to the b-pillar by tacking on some metal tubes.When removing the clip cut the bottom of the a-pillar first to stop the roof sagging.Support the bottom of the clip with whatever you have,blocks of wood or jacks etc.Get cutting.Hardest bit will probably be cutting the chassis legs away,so come in from the lower front valance and cut windows in all 3 layers around the chassis and tidy it up afterwards.Obvioulsy measure everything before cutting.I don't know what you front arches are like but mine where shot so I just hacked them off.Drill out floor pan spots welds from the front(remove front panel skin)
When you do go to refit,make sure that you have got the point where the chassis rails join the front clip trimmed enough so as to not make it stick to far out.Tack everything on to start with and then fit a door on properly and make sure all the lines around the door are good.This will tell you it's in the right place,if not re adjust before welding it right on.