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I'm not sure about the EBAY units but I have installed three snorkels. They're pretty much the same but for reference I installed two Safari snorkels one on a 60 series, one on a 73 series and now an Ironman one on my LX. I noted a mpg savings on the 60 and 73 but they were diesel. Not really noticed anything different on the 80.
Use a center punch to mark the holes you need to drill. The punch will keep the drill bit from wandering.
http://www.harborfreight.com/catalogsearch/result?q=center+punch
Drill pilot holes
Buy yourself a step bit - they are designed to work in sheet metal
http://www.harborfreight.com/3-piece-titanium-nitride-coated-high-speed-steel-step-drills-91616.html
Also get the correct sized hole saw - your kit will give you the dimensions:
http://www.harborfreight.com/catalogsearch/result?q=hole+saw
The kit should also have a template that you tape onto the fender. A good idea is to check it on the snorkel itself to see how accurate it is. Every single one that I installed, the template was slightly out. You might want one of these if you have to adjust a hole.
http://www.harborfreight.com/5-piece-rotary-rasp-file-set-4476.html but pick one that is suitable for metal. The ones in the link are for wood but I couldn't find a good example. You get the idea I'm sure.
After you drill all your holes, spray some primer in a cup and use a q-tip or other small brush to paint it on the exposed metal. Deburr as well before you prime. Metal Primer is porous and will eventually allow rust, so do the same thing with a finish paint that you put over the primer.
You will make your life easier if you remove your antenna as it in the way for getting at the nuts for the snorkel. You can re-install the antenna after you finish the snorkel. Use some thread locker on the studs that stick out the snorkel. You may also want to remove your running light beside the headlight as you can reach up inside to access other nuts.
Have some RTV silicon handy and use a small amount on all hoses that connect the snorkel to your air box. Some air boxes have a clean out trap on the bottom and you'll find this facing the grill. Silicone it closed.
I've always used rivets on the A-Pillar. I've used RTV silicon around each of the holes I've drilled before installing the bracket just to make sure water doesn't get into the pillar.
There are better tools than ones at Habour Freight - just saying.
I've covered everything on the fender that I don't want damaged in several layers of masking tape just in case you slip with the drills. Don't force it - let the drill do the work. Firm easy pressure. The hole saw will wobble with too little pressure though.
Carpet removal for those times you go wading is optional