If the old tank is still attached, I'd draw pairs of lines that intersect over the centers of each existing hole Then drill at the intersection.
That's how I did it on my RV-4, more or less...
1. Get a compass (for drawing arcs) that has a screw adjustment. Set it for maybe 1.5", don't remember;
2. Put masking tape on the wing just aft of the tank;
3. Put all the screws in that hold the tank in place. Doesn't matter if the tank is there or not. Adjust the screws so that the indentation in the screw is at the same height as the skin of the tank would be;
4. At each screw, with the pointy end of the compass centered on the screw, draw a part of an arc on the tape, one inboard, one outboard;
5. Put tape on the new tank covering where the screw holes would be;
6. With the new tank held in place, draw intersecting arcs on the new tape. It doesn't matter where on the old arcs you put the pointy end of the compass. This will locate the new screw holes accurately. For best accuracy, the arcs on the new tank should be close to perpendicular.
For the screw holes not along the spar, do the same thing.
Easy enough to try this with a piece of scrap and maybe four adjacent holes.
It's really not that big a deal and works really well. I didn't say that it was a lot of fun...