Since you said you were new to this, I'll speak candidly.
Slant 6 = boring, no matter what you do to it- IMO they are garbage as previously stated.
G3 = lots of challenges but can be overcome now with support from vendors like Holley, TTI, etc. Just getting a G3 in an A body can be a challenge, all of which are well documented in this forum. It won't be an easy drop in. After you mechanically get it in, you will have to overcome the EFI, if you chose to keep it. I don't know your level of knowledge on EFI, but it can be a lot for someone that doesn't understand it.
383 big block = probably the easiest big block to put in an A body since it has the shortest deck height of all the big blocks. There are adapter motor mounts and headers that will simplify things. Parts are abundant for these things both new and used.
Any V8 swap will require a new transmission and likely a rear end since your car probably has a 7-1/4".
As with any project, you need to decide what you want your end goal to be. Also realize the car will be down for a while you are doing the swap, especially since you are new to this. Again, I don't know your mechanical ability, but it's easy to struggle when you're inexperienced.
