I have an 1987 Monte Carlo LS with the 4.3 TBI motor in it. GM G-Body based car like yours. This is the one that really got me into the car hobby. The 41 Chevy is a father son project that came later.
As for engines first ask your self what you want to do with this vehicle. The G-Body platform had different v6 and v8 engines applied to it over the years. Cruising around or performance. Keep it stock or hotrod. Budget?
The cheapest option is to replace the engine you have with the same one. If everything else in the car works the replacement should bolt in. No adapters etc.
Perhaps the most popular one I see installed that was not originally offered is the SBC 350. It is the same physical size as the 305 that was originally offed in the car. There is allot of information on this install on the internet.
My preferred engine is the 4.3 TBI. Nice motor for cruising around. It is not a powerhouse but has enough torque to get the car to speed without issue. The TBI means no messing with a carburetor. My car has had the same 4.3 in it since it left the factory. Anything with fuel injection may be more work in yours since it was originally carbureted.
Do not forget that if you put in a different engine you will need to replace the exhaust and make sure to make clearance on the drivers side for the steering shaft etc. Depending on the size of the engine and headers you may need to remove the heater box from the passenger side of the firewall. There is a delete plate for this.
If getting a used engine check the reputation of the source you are getting it from. If it is from a junkyard the quality of the engine is based not only on how it was removed but how the prior owner cared for it. If you plan to keep the car for a while then getting a motor from a rebuilder with a warranty might be a safer bet.
Remember do not get discouraged during this process. It is common that all projects take more time than planed and require more money than originally budgeted. There will be little things like bolts or tools that you need to complete a task that you do not have or did not budget for.
With the G-Body cars get a good set of metric wrenches, both ratcheting and non-ratcheting. Also a good set of sockets, 6 points are highly recommended to avoid rounding nuts. At times you may want two 10mm wrenches and sockets. Do not loose the 10mm when working on this car. A special tool is needed for the water pump pully. Get a torque wrench to make sure everything is tightened correctly. The Haynes repair manual list all of the required torques values. The inexpensive Harbor Freight ones are fine for things like the water pump etc. Tekton makes some nice long handled ratchets. I have one of the 3/8 long ratchets with the 18 inch handle. Works great for breaking loose bolts and not having to reposition each time. https://gbodyforum.com/
is a forum dedicated just to GBody GM cars. There is a wealth of information on this sight specific to these cars already posted in existing threads. Placing a question normally gets a fast response. https://www.mikesmontes.com/
is one of the best sources I have found for new, NOS and reproduction parts for these cars. There service is good and the prices are fair.
Hope this helps.