I know the feeling
I have a 92 Caprice and I swapped engines with a 69 Chevelle 350. In doing so I neglected to reconnect any computer lines to the engine. I was running Blind basically, I had the exact problem you do, at higher temps it sputtered and jutted the engine even died a couple of times. What I did was replace the thermostat and get a flex fan. The Fan runs at the speed of the engine so it keeps it cooler even when in idle. Hope this helps.
PS: I am trying to locate a LT-1 Engine where did you get yours?