[1971WARBIRD]

one of the stories I heard(and you know how that goes). the HO was a great engine for the street, with lots of power down low. when guys started running them on the track and winding them out, the rods were not up to the task. miss a shift and out the side they would go. even on the street, if you tried driving it like a 400, it might decide it didn't like it and take a break.
I will let the people that know the system inside and out, get into the details of how it worked, but remember. back in the day, they didn't care about numbers matching blocks and such, just if it ran right. I knew of guys who took their cars into transmission shops to have them rebuilt, only to find out much later that the shop had swapped in an already rebuilt transmission for theirs that needed to be built.
so in other words, yours may have needed new bearings, new head gaskets, what ever that we would call simple now, and the dealer just drop in a replacement block. when Murphy Pontiac went out of business in Troy Ohio, they still had a couple 4 bolt 400 fitted blocks in the parts department.