Another method I have used with great success is to cut the existing fender 3/8" longer where ever it is to be joined creating an overlap on all sides of the new panel. Then using a Flanging Tool (Harbor Freight / Northern Tool - $30-$40) flange the car side of cut off areas on the car. What this does is create a recessed lip so when you lay on the skin the new surface is almost perfectly flat so there will not be 20 lbs of filler on a NEW panel.
Now also consider a couple of more things here, leave enough space at the cut areas all the way around so when you weld the edge of your new skin into the recessed lip you are still "at or below grade" on the surface of the panel. this will minimize copius amounts of bondo and make your patch stronger with the lip the flanging tool creates - small as it is. Secondly, when welding on this skin or any big piece of work, do not weld continuously anywhere. Weld the size of a small tack on spot and move completely to the other end and tack, imagine a clock face and start at say 12, jump to 6, recheck everything and tack at 9, jump to 3,then 7 then 1....... the point being no one spot is getting a lot of heat. take you time this is not a 2 hour weld job, if the whole panel warms up stop and go do something else for 30 minutes, no direct fan either, uneven cooling/heating spells warpage and ultimately= filler. grind down the tits on your tackwelds, again controlling heat - grinding is friction and gives off heat and rust proof then do bodywork and no one will know anything. also the flanging tool makes the insde of the repair much less obvious. On replacement panels it is imperative you trial fit nad make the new panel fit tightly everywhere, some of the key lips are usually under bent like the 1/4 ti rocker fit - make it tight. Lastly take plenty of pictures so when the car show or sale time comes you can prove the quality of your work.
Also you really need to make a template for all the key areas where you have to meet other body lines, use anything - wire, tapemaesure for sure, if you are cutting out the 1/4 where the ribs in the door are - measure everthing point to point. I mean to say from the lower door rige to the highest crest on top of the fender and so on, if anything is shortened, extended, wrong place it will look horrible. Set up and match up ten times if you have to , weld once and be happy. if a 1/4 line is out and you move thedoor to match then the fender will have to move too and the hood, wiper grates, on and on andglass,striker, weatherstripping.
Trust me it will hard if not impossible to fix. Sorry if I yapped too much
best of luck, Joe