LS engines have been fitted but I don’t what mods had to be done. Hopefully someone here who has done that will fill you in.
When I had a ‘70 Avanti II I bought a crate engine…350/330 hp version. For my style of driving (I’m not 25 anymore)…I think it was overkill. The 290hp crate engine would have done just fine plus it’s a regular fuel engine where the 330hp crate engine was premium fuel. With a 700R4 transmission it could spin the tires in any gear.
There was minor issue with the 700R4…the transmission support had to be relocated somewhat and a new neutral safety switch/backup switch assembly had to be fabricated. No big deal but was necessary. You could go with converting it to a manual 5- or 6-speed transmission. I’m kind of sorry I didn’t do that but it would have cost more to do it the I would want.
A 200R4 engine would actually have been a better choice…a better spread of gears and could have been built to 700R4 strength…plus being smaller.
I also went with an aluminum driveshaft…less rotating mass and un sprung weight. I also used lightweight composite leaf springs for less weight.
I went with an Edelbrock EFI which was trouble-free and performed superbly. Sticking with a carburetor would have certainly cost much less but I wanted EFI.
If I had to do it again I would gone with electric power steering to get rid of all the hydraulics and hoses hanging down under the front but aftermarket electric power steering was just hitting the market at the time and I didn’t care to be a test case using it.
Overall…the best I can tell is to follow the KISS principle…Keep It Simple, Stupid. Trying to do too much too fast can be counterproductive and expensive. I would suggest starting with the brake system and suspension first. Make sure the hog troughs are solid as replacing those eats up budget very fast. Then the drive train and cooling system. Interior and body last.
Make a plan and stick to it. You can get into the weeds faster than you think with mission creep with any car but more so with an Avanti.
Rebuilding an Avanti is an act of love…not of reason.