Well for what it's worth I dropped the LS1 in my car on the 4th of July. It took me until August 12th to go for the first drive. I only have my evening's and weekends but I put alot of time into it.
I assume that you will be deleting the power steering pump so you will need to modify that.
Also you need to come up with a clutch cylinder and improvise a way to mate up to the T56. PM me and I'll show you how I did mine, and a new way over on LS1Tech.com
Just to get the car ready took me 2 months. Engine removal, engine bay cleanup, Tranny tunnel mods, Fixing up a throttle cable (I used an LT1 cable and it fit fine). Fitting and wiring an electric speedometer and tachometer. Running all the needed electrical for the fans and fuel pump.
Take your time estimate and multiply it by 10. Then take your expected cost and multiply it by 2 or 3.
The exhaust will have to be custom. You will have to weld on O2 bungs since all the aftermarket shorty headers dont have them.
You have to have a good fuel pump to feed that hungry monster at 58 psi. If your not going to install an LS1 fuel tank then that means you need an aftermarket pump and regulator.
You will need a good wiring diagram of the 280.
Oh yea, remove the front bumper when putting the engine in or you will really have to fight it. That plus raise the whole car at least a foot off the ground with jackstands.