Hello and welcome to the forum enjoy.
I put a 350 cast iron into a 75s troopy using
a Marks adaptor kit,
One make sure your engine mounts are in
the right spot and two check the fly wheel
The one in my kit weighed about 35kg guessing
where the std was only 11kg,
LS3 would be different but check just in case
maybe even lighten it a tad.