Kyle and Bladed_metal,
I've been in Celica community for a while, not long, may be 7 years or so. I've owned 1 GT, 2 alltracs, and bought 2 GT Celicas for friends. During this time, I've seen tons of people said they are going to do AWD conversion on FWD Celica. I'm not saying it's impossible, because I've have seen 2 6th generation Celicas with AWD conversion. Both of them were for sale at the end. One of my Alltrac friends tested drove it, and he dislike it.
I've yet seen one 5th gen Celica with AWD conversion. Personally, I think it's a bad idea. However, I'd really like to see it done. So, DO IT, if you have time and money. It's your money, not mine. I just gave out my $0.02.
At the end, you are taking risk. Think about unfinished project, ruining the car that once you can not bare to get rid of, debt, sell the car to cut your loss, etc...) Remember, I'm not trying to dicourage you. I'm just trying to inform you what I've seen. I wish you luck and please keep us update on your project.