I want to possibly pull a small trailer from PA to California with my Bonnie...if possible. Will my car be able to pull say 1300-1400 pounds including the trailer? for $3000 miles? Over the Rocky Mountains? Can my transmission handle it??

it should be able to. I believe the Bonneville is rated for 1,000 pounds and you're not over that limit by that much. Just make sure the tongue weight isnt' more than 200 pounds or so. My biggest concern is that you may be under boost for extended periods of time. If you do do this, make sure you drive in D, and no OD!!

You should install a large additional tranny cooler too to keep the temps down.

I agree with custom88, although i would reccomend at least a fluid change before/after to minimize your risks (a trans cooler also helps).

We towed a small popup (weighed about 1300 lbs fully laden, 170lb tongue weight) with our 98 LeSabre from San Diego to Lake Tahoe and back last summer. We had one of those ugly roof-mounted clamshell cargo things pulling drag, too. Had absolutely no issues with tranny, and fluid was perfectly sweet and red afterwards.

We are planning to tow the trailer to Colorado and back this summer, and I'm planning an ATF swap and tranny cooler install before that one.

A tranny temp gauge may not be a bad idea while you are at it. Can never be too careful.

I agree with the tranny cooler being a good idea if you dont already have one.

Alot will depend on the weather/temps outside.

I'm one of only a handful here that tows. My hitch is now on my 95. I can tell you NOT to do it without a BIGASS cooler. And I strongly suggest a trans temp gauge also.

are the extended periods of boost a concern at all?

Boost creates heat and burns more fuel. Other than that, no. Keep that octane up though.

I can tell you from experience, Aero drag will have a far more damaging effect on the trans and temps than the weight of the trailer.

How much weight will be in the car?