upgraded from v-6 to ls, what size radiator do i need to buy

Mine is probably 18 by 36, but I can measure when I get home to be sure, it also has 3 Spal Fans..

Depends a bit on how much power your engine is putting out and how much load you intend to put on the engine routinely.

Also, I would say it depends on the quality of airflow you have to the radiator.

Without trying to over analyze those items I would say get as big as you can possibly fit into the car. Also, put the effort out to shroud the radiator. Look at a newer model domestic truck and you'll see what I mean. They put significant effort into ensuring that the air goes into the radiator instead of around it. Leave the sides bottom or top open and the air will go around instead of through.

Utilizing a standard dimension from the radiator builders will save you a grip of cash. Custom sizes go off the chart quickly.

I would also look to get the best, highest CFM fans you can get.

IF it runs too cool you can always restrict the flow or control it with a thermostat.

Remember that if you do not run a thermostat to block the water bypass hole behind the thermostat itself. Generally a pretty big hole, 1 to 1 1/4". Plus put a bleeder hole in the bypass plug. Do a search hear and there are some threads.

For my BIG car that will run a windshield and an LS2 I have a 19x36 with 2 16" Spal high flow fans. Air flow around it is pretty poor since it is not well shrouded in. Until I blocked the bypass hole it would run hot at times windshield in or out. Put an Evans pump and their coolant in it and it helped some but did not fix it until I got the bypass hole plugged. Now all is good and have not got it to run into default yet with windshield in.

I've built several V6 cars (Honda) using the late model Jeep Cherokee radiator you can get at most radiator shops, about 99.00 for 2 core and 149.00 for 3 core. It's about 15" X 30" Both work fine with a couple of fans and a simple sheet metal cover with holes to mount the fans. Use the 3 core if you are going to run in the heat of the summer.