Ok my grand dad worked there this how it got explained to me.
They were all cast with the high nickel and extra webbing of the 390/401's but had the bore size of the 290/304.
Than they were bored according to what was needed.
I asked about the Tall deck versus short deck his response was:
While boring to fit the CID need they got decked down if need be at that point.
You could get anything from a 290 to a 401. Your 390 n 401 were on par but the smaller engines got the extra benefits of the larger engines, If you got a service blocked 290/304 or 343/360 they were structurally stronger.
:e did make reference to the racing guys it wasn't more fudging CID as it was compression, camshafts, and so on he said a quick skim down the block and your 8.3:1 360 was now a 10:1 360 swapped the Jeep cam for an AMX/Javelin cam and you got a "stock" 360 for a 70 AMX getting put into an 80s Jeep mud racer.
Now he did mention the crank shafts were easily interchanged so a block stamp 304 could easily become a 401 tho ya lost structural strength it only had to hold up for a weekend not 200 thousand miles
So without him directly saying IT, I think he had helped some guys do a little cheating.
If NOT it did give me the impression it did happen or it could have happened with ease.
AM-G