Hi Quin,
I will try to describe the procedure as I have changed all the panels on my 280SL 1970 (i.e. :front end, front long bar structures inside the front fenders, front fenders, rear end and rear fenders).
I've some picture if you want.
If you don't have to change the front end, before to disassemble the front fender, you have to put the brand new front fender on the old one (still on your car) : then you will know where to cut properly the old fender and you will have the welding line at the right position for the newone (between front end and fender and between windscreen bay base and fender). As you maybe know the cut as to leave a little bit of space between the 2 parts to be welded because the metal part are increasing in volume during welding and you have to leave space to make the welding material going into the gap.
If you have to change the front fender I assume that you will also have to change the long bars inside the fenders (I don't remember the exact name of these parts) because they are often rusty on their top (where mud was staying). You will have also to change the 2 headlight bowls. These are particularly difficult to assemble on the fender and front end as the parts on the market do not match exactly but if you have one good body restaurer it will be ok (one tip : when the welding of front fender to the front end were achieved, we put into place the 2 bowls and before to weld them, we screwed them wit self tapping screw. Be careful because you will have to add "lead" / "tin" on the perimeter of the bowls (I saw that after the car was painted!!!) to be sure that the surface of the aluminium headlight structure is fully maching the surface of the bowl perimeter. I assume that MB assembly line had some dies to be sure to get the final geometry...
As regards the 2 long bars, you have simply to put the new ones at the place of the old ones an before welding (by spot welding) to be sure that the inner fender is in good condition (where the bar has to be spotted). Before to spot check the diagonal dimensions (upper/rear external point on left fender place near windscreen bay / down / front place of right fender and then the opposite).
When the bars will be spotted, then apply the two fender as blank assembly and also put the bonnet in the middle to check if you have enough space to put this bonnet. You will maybe have to erase the sides of the bonnet at the end (put one wool cover on the engine to place the bonnet in right position because you cannot use the bonnet fixing at that time).
It is better to dissassemble the windscreen as the front fender is welded up to the windscreen frame (except for 3/4 front fender panels).
Please confirm if you need some pictures.
Best regards.
Vincent R (France).