it would be a good idea to put it in the garage, even with a vent. I had a propane generator and kept it in its own little house that was only 3 sided.

They probably can be, but this is NOT a job for somebody without the proper credentials.

Better to put it inside a little tiny shed built just for it - but again, only done by a professional.

Propane tanks must be outside.
Depending on how big, you might want to think about outside venting for the heat they generate.

still have no power and don't want to get stuck like this again.

I am still trying to figure out how much wattage i need to run my 4-ton florida heat pump, and well fro the geothermal.

ideally i would get this one http://www.electricgeneratorsdirect.com/Briggs-&-Stratton-40243A-Standby-Generator/p1979.html and plumb it in to my natural gas... problem is if i put it outside on the highest point of my property it will still need to be elevated 4 or 5 ft to avoid being flooded if we get hit by another sandy

I live in Louisiana, and we bought a 20KW Generac generator from them 2 years ago. We ran a natural gas line to our house to run it. (House is/was all electric) Best investment we ever made.

We built a 3-foot high platform off the back patio to place the generator on. Has worked like a charm.

6x6 posts and 2x6 planking. We have the transfer switch box (the weatherproof type for outdoor mounting) on the exterior wall of the house, in back of the generator and platform. We don't have a cover over the generator, and have not had any problem with it at all.

i suppose i would be better off building a platform and keeping it outside

After going through 4 hurricanes, sometimes going 2 weeks+ after the storm without power, we finally caved and put out the bucks for ours. We will never regret purchasing it. We still ask ourselves why we waited so long.

any engine will scavenge for air w/o it.

I see you're worried about flooding. Just as an idea, maybe put in a patio cover but make it sturdy enough to support the genset? Put it on the garage roof, maybe?

You really, really do not want this indoors

i could use double-walled pipe for exhaust

There are some big (as in hospital) sized generators are located indoors. Maybe there is a residential standby version?

If a generator has the wrong fuel mix (say you accidentally leave the choke open) or the oil is dirty it could blow flames out the exhaust like a blow torch (mine did this with the choke open) and burn your house down. Even without that the danger from carbon monoxide would be too high to risk it.

edit - just noticed you said gas or propane and not gasoline, but am leaving my response up for those thinking about gasoline powered generators. I also imagine that no matter how it is powered the carbon monoxide danger would be just as high

Your locale may have codes regulating the installation of such. Unless you have a great deal of experience yourself, all non-portable gas fired appliances ought to be installed by a licensed professional.