You could do that or your you could do this...
For the spoiler take some grease and put it on the bolts for mounting insert them through the spoiler then set that thing on your trunk BAM you now know where to drill....find a good close sized drill bit and drill them out... now for the old holes take a piece of aluminum (even a cut piece of coke can works) flatten it out and go to your hardware store youll need these things...

Bondo and a tube of hardener
Primer your color car paint and clear coat all in spray paint cans
JB Weld Liquid Steel

Now sand a good 2 inch patch around each hole and clean it really well, JB weld the pieces of aluminum you have found to the bottom side of the old holes for your old spoiler (not on the outside of the car on the inside) and wait on that to dry (overnight) once its dry make sure the aluminum is properly welded up if you see any unwelded pieces mix some more JB and fix them. Now mix some bondo not very much hardener at all about a teaspoon of bondo only needs a little squirt of hardener, be quick because that shit drys fast and patch the top of the holes where the spoiler was (this time bondo on outside of car)wait an hour or two for that stuff to harden good... Now sand it smooth and even with the car and clean it off. Patch off the spot to paint with some masking tape and cloths and prime the entire area only one coat. Wait 3 hours or so for it to dry and then paint the main color on 3 coats 30min drying time between coats and then wait 3 hours for it to dry. Now take a good clean coth and buff the ever living shit outa the paint you painted on till its shinny as crap. Now coat it 2 times 30min between coats with the clear coat and after 3 hours rebuff. It will look like a pro did it if you dont messs up if you do its a learning exp and not that hard to fix. Now for the body kit same painting proceedure applies but make sure you fit the kit well if its fiberglass then just bondo the small cracks etc if its urethane then you have nothing to sweat over.

BAM! you just mounted your own wing and kit! ...remember when you paint do it slowly and dont EVER stop moving your hand use small sweeps of spraying and only coat thin dont glob it on or it will run. Keep your hand about 6" to 8" from the item your painting...This is a trial and error deal the first time you paint something but just stick to these tips youll do better than you think and by the way use Dupli Color Spraypaint it is very strong and it's a high quality paint. It won't chip and it will match your paint color better than cheaper paints.

I have done my own and countless of my friends body kits and wings and even some custom fiberglass molding with spraypaint (just take your time).

You can get this done for about $50 in supplys and a good 3-4 days worth of hard work.

198hp @6350 rpm NA 420A (Thanks to new JE forged 9.5:1 Deep Dishers ready for Zex Nitrous 150 Shot) -THIS IS NOT MY CAR ITS A FRIENDS!