I assume you meant to type 481988 for the block casting number. These were used in '71 thru early '75 model cars. Not used in '70.
The date code looks to me to be A041. if this is the case, it was cast Jan 4, 1971. A block with that date code would likely to have been used in a '71 model car.
Looks like a YS code 481988 block could have been used in a '71, '72, '73, or early '75.
Wallace Racing's Pontiac Engine Search
The head ID numbers would be cast with the head, over the center exhaust ports. '71 400 4-barrel heads would probably be #96
. But, most any Pontiac heads could have been bolted on, at some point in time.
The 1st pic of a #96 head, shows a date code of L150. So, it was cast in late '70 & used on a '71 400 engine. The 2nd pic is a #96 head with a B191 date code, cast in Feb of '71. It was also used on a '71 model engine. Don't know how many different months these were cast in. But these heads were used only on '71 model year 400 engines.