First, ABS works super in todays cars. No amount of brake pumping by a driver can simulate what an ABS systems does. Not even worth debating.
However, the ABS system of this time period, IE: early 80's I'm sure is less effective/sophisticated. But, adding this system I believe would be a very large endeavor. Hardware and electrical would be quite an undertaking.

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John