I'm confused why you think the ls1 is a more direct swap???
If you want more displacement and better off boost torque import an RB30 block.
With the RB assuming as you say you can use the existing transmission, you won't have to worry about transmounts/custom driveshafts, no cutting firewall/transmission tunnel (Assumming the RB25 bellhousing is no bigger then the z32). So all your talking about is, engine mounts, fuel lines (May not need re-routing), exhaust, intercooler/piping.
LS1 as Braap has detailed will require firewall modifications, transmission tunnel modifications, you have to modify the inside of your dash and loose some of your standard accessories, you will have to look at different exhaust headers or modify the steering rack, etc, etc, etc.