Looking at pulling aluminum gooseneck stock trailer with:

'99 F150 ext. cab, full bed, 5.4L, 3.55, trans cooler, 7-wire harness, 4 whl disc/ABS, no chip/programmer, no cat-back, no K&N etc.

Trailer is 4800# empty,
most often 1-2 horses, plus tack/hay(total 7500#) for 120 mi. one way, then back again,

max wt ever will be 8000# in emergency like evac for hurricane.

Have F250 for full cattle loads, but it is a single cab, not very good for hauling clients/horses at the same time.

Your thoughts/opinions are welcome. Thanks.

 

you could pull it, I wouldn't make a habit of it, but you could do it...
The biggest limitation is your gearing.. 4.11's would really help.

 

I suggest you take a 120 mile ride in the back seat of your F150 to see if you really want to stuff your clients back there. Maybe it's time to upgrade to a newer crew-cab F250/F350 and write it off as a business expense.