As I age I forget things , one is what determines whether you use a thin or fat front seal on oil pan to timing cover? I know the change was like 75 model year,but what changed,the pan itself or the timing cover?? I need to buy a 1 piece pan gasket but my pan is a new replacement used with the original 1970 cover.I compared my cover to a few chrome covers I had laying around and the lower lip area looks the same.Any rule of thumb or quick way to I.D. what is needed??? my other option is to buy 1 of each and carefully fit to engine and see which fits [last resort]thanks in advance!!! Steve!!!!

It's the oil pan that determines what gasket you need.

I can't remember the years right off hand.

But the older engines used the thin front seal. Then around the mid to late 70's GM changed the oil pans and those needed the thicker front seal.

Also alot of the aftermarket oil pans needs the thick front seal. Like the summit 7 qt., the milidon kicked out pan, etc

I have a block from a '73 PU and used a different pan and timing cover. I used the thicker one piece gasket and it worked out great. Has not leaked since I put the engine together over 2 years ago. I think it was the late 70's when that seal changed from thin to thick.

Early front seals from '57 to '74 are thiner than the late model ones.
to find out which pan you have,lay a straight edge across the top of the pan,then messure down ,the early ones are 2-3/8
the late ones 2-1/4