Another way to tell is that the Boss block will have 4-bolt mains and taller deck height. Also, if the engine has been untouched, then it should have 5.155" connecting rods and 351C heads.
I have a friend that just rebuilt an engine w/a Boss block so he really knows his stuff.
E
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1991 5.0 LX Coupe - 40,750 miles
331 cu. in. / Tremec 3550 / BFG Drag Radials
12.22 @ 114.31 mph - w/1.89 60'
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