I put the
oil temp sender in the sump. On US crate motors there is a threaded boss on the side of the sump where the sump heater screws in. I just machined up an adaptor to plug the sensor in there.
If you are game to unbolt the sump there is enough meat in the side of it to drill and tap a hole for a sender. Then you will be measuring the temp of the
oil directly in the sump. It's dead easy to unbolt the sump. It uses an O ring gasket that is reusable.
As for the water temp sender, I've seen some people drill and tap for a sender into the top of the water pump. Some models of pump have a flat boss in the casting there ready for a different style of water neck. It makes a good spot for the sender. LS1 thermostat restrict water into the motor rather than out of and the thermostat is in the suction pipe of the pump.
I used the blocked off hole in the drivers side head for mine but drilled it out larger for a mechanincal sender (a whole 'nuther story). Stock it has a 12mm thread and it'd be easy to make up a 12mm to 1/8NPT adapter and that would require no drilling.
Cheers