We just replaced our 93 subs fuel pump today. It took us most of the afternoon, but all in all, not a hard thing to do. Mostly just time consuming. And dirty!
We also had quite a bit of gas in it and had to siphon it all out. Filled our boat and our other truck.
Our only problem was that the problem wasnt the fuel pump! Oh well. At least I know the pump is good for a while!
As for space, it didnt seem to crowded to me. Im small, but my husband was under there with me and he is 5'11" and @ 210lbs. He didnt have a problem fitting. We did use a jack with a nice board across it to slowly lower the tank so we didnt rip anything electrical out. I would suggest a 2nd pair of hands for pulling the tank and putting it back in. Just to make sure you dont drop it and ruin something else.
Good luck!