Will a 93 Buick lasabre engine Fit a 99 Grand Prix? Both engines are 3800s.

The 3800 series ran from 1984 till 2008 and those motors were re-called in 2009 with major oil leaks.
So, you have to be careful.
The only information I could find or have on a comman and easy swap would be the 1995-2002 3800 Series II L36 engine.
The Engine prior to that is a Series I L27 made from 1992-1995. The engine that saw the end of the 3800 series was the 2002-2009 L26.
The L36 1995-2002 is a great motor and won "WARD's Top Ten Engines" 1995-1997.
You might be able to use part of the block, possibly the short block, from the 1993 motor, but you would probably have to take a lot of parts off of it and transfer them to your engine. Your motor has a rated 205 HP while the 1993 engine is rated at 170HP. So you would be losing some horsepwoer and it is hard to say how many parts you would have to switch over for the motor to work with your car's computer. The motor in your car is lighter as well and the older motor would affect the front suspension.
So, I would not reccomend it. There are so many of the 3800 series motors out there that a little research on the internet could get you one and the swap would be easier.
Even if you get the same motor, if it comes from a different car you still will probably have to switch some parts like the brackets that hold accesoiries such as; power steering pump, A/C compressor, etc. And the motor mounts could be different. It gets tough even at that level as brackets that may look the same can be a centimeter different and you won't notice it until you are bolting up the motor in place.
When you look for a used motor, a reputable salvage yard should be able to tell you exactly what is an easy swap and what parts you may need to switch over or maybe even have to get with the motor you purchase. You can find good refurbished motors on the web too and then you would just have to swap some of the aforementioned parts over, but there would not be any guessing game. This is because you would have the block down to a long block which is a motor with the heads on it. It does not always come with an intake manifold or other brackets or sensors and you would have to get them form your car.
So, it is not easy to do a swap unlesss it is literally from your same car. Even the years may be a little different but that would be much less of a hassle as you would know that at least the motor mounts and brackets are the same, but maybe an emissions part or sensor could be different.
To answer your question, it is not a good swap as it would be more trouble than it is worth and you would be losing Horsepower and gaining weight which would cause it to handle a little different.
I took some time to look on EBAY and found the engine. I would not buy through EBAY, but you can go to most EBAY stores and take the name of the store and search for it on the internet and see if it has a real internet site and sometimes you can get the phone number from EBSAY as well. Doing this you would know the true source and deal directlky with the shop and they will give you the EBAY price in my experience. This allows you to research their reputation on the net.
One really good source of information would be to join a Grand Prix Enthusiast Forum on the web. It is a group of owners that love their cars and do a lot of their own work and have dealt with problems that you would not find information on anywhere else as they will post what they found. They are usually good step by step right ups and even have pictures. They really have information you can't find elsewhere and you could even get information on who is a good salvage motor dealer to purchase an engine from and information on the kind of work involved with swapping different motors. It may take some time to search the site or post your question and get an answer, but the people are usually very helpful as they are into their cars and like to share information.
So, look for the 1995-2002, L36 Series 2 3800 engine for your swap. You can use this web site's search engine to find reputable used engine dealers by simply searching the term and you will see links appear on the page that comes up. Anytime you even mention a word like, used motor, in a post and send it, you will find that links are ut up for used engine dealers or other related items and they are location optimized to be as near to your location as possible.
So, good luck and if you have any more questions about this, let me know.
Take care.

You are very welcome.
Comeback anytime if you have another question or just need advice.
Take care and good luck in your engine hunt.

Cr. C