alright ive spent my fair time doing the search and came up short.
so i have to ask. i've seen it a month or so back, but i cant find it now.
i know its possible to do, but are there any modifications needed, are they also 15.5 gallons?

reason being, i have to install a new sending unit in my current tank, so id rather just install a new tank itself, if possible, a 4th gen.

 

I think theres a tech article somehwere on this ive been wanting to do the same thing I just havent been able to find much on the swap, and if its easy or not I know for sure theres a couple people on here that have done it though

 

Search for "4th gen tank" and you will get plenty of information:

-4th gen Gas Tank into a 3rd gen?
-LS1 Gas Tank
-LS1 Fuel Tank (Plastic 99-02 Style)
-4th gen Gas Tank Swap

In short, no major tank installation problems, but there will be other issues to deal with.

I wouldn't do the swap if you don't have a LS1. It will be more work than it's worth to modify the fuel lines and wiring just to adapt to the stock setups, and the weight difference between the two miniscule (I'd guess only 2-3lbs, as I've handled both). The sending unit is also different (40-250 ohms vs. 0-90 ohms) and so if you want the correct ohm reading, you'll have to spend an extra $100 for a new sending unit from a Grand Prix.

I would just install a new pump/sending unit and call it a day... you'll have plenty of work replacing the stock parts as it is, without having to figure out how to connect the new plastic push lines and modify the wiring.

1998 LS1 - 15.5 gallon steel tank
1999+ LS1 - 16.8 gallon plastic tank