considering you have a 91, and the vortecs came out in 96, there would be some work to be done.
you would need all of your belted accessories for the engine, ac/alt etc.
and with it being a vortec going into a 91 you would need to do alot of extra wiring etc.
in my honest and utmost opinion, I would say try and trade off the 91 on a 96+. they can be had easily for around ~$4k on dealer lots, but cheaper from private owners. Do some looking around, if you were to trade in your truck and if your truck is paid off you may only need to dish out another 1500-2k for a Vortec, which is probably close to what you would spend trying to fit a vortec in your 91. Also you would be able to get a newer truck, a lottery possibility of a 14bolt, and quite a bit better color combinations.
Also, I would suggest going with an XCab, you may end up liking it and never go back to a regular cab.
My $.02