That depends on your pan. Somewhere around 1974-75 they switched from the "thin" to the "thick" pan gasket and ALL of the difference was due to the change in the pan design around the front "half moon" arc.
If it's an aftermarket pan, it always takes the thick gasket.
1990 454SS pickup. Sold, but I think she found herself a good home.
78 Malibu. Inherited with only 35K on the odo! Mild 383 with Weiand 142 blower on top. Mid-high 12s on street rubber, bottom 12s on DRs with the trans left to shift for itself. Runs so good I may never drive it again.
"Last remaining QJet tuner on planet Earth!"