I did that in my 70 Skylark. I put a big block Chevy in mine. I even went as far as to give her the "chevelle paint job!" hehehe You can see it in my photo album. Now this was about 12 years ago, so it has been a while, but it wasn't all that hard at all. I had to move the engine forward a little bit so the new chevy motor mounts were at the edge of the cross member. I also had to get the drive shaft cut and balanced. At the time, I had an HEI ignition in it so the HUGE, LARGER THAN LIFE distributor cap was touching the firewall which caused me some problems. Save yourself the trouble and don't go with the big dumb HEI cap.
At the time, I also did not have the engine "built up." I just bought it off a buddy and dropped it in, so I was still running the stock manifolds. You will also have to re-route your fuel line as the fuel pump on the chevy is on the other side. You really shouldn't run in to many problems as your Skylark is an A body like the Chevelle. (That's the part that puzzled me when I couldn't just "drop it in and go." But you can do it! It's not that hard at all. I can't think of anything else, but if I think of any other snags I hit, I will let ya know! Good luck and enjoy!
Oh yeah, I'm sure you already knew this but you will need to get a new tranny to bolt to that Chevy, unless you have one of those universal bell housings. (I had one years ago, but got rid of it like a big dummy!)
If you have any questions, feel free to send me a private message or e-mail.
Rock on and God bless...