... but the published data we are all finding doesn't show the "CLA" used in '78.
So which is it?
1> incomplete data
2> transplanted block block placed in a '78 and restamped with '78 vin...
3> ???
...
Most likely
1 or
3 
No reason to restamp a block then ...
I have personally run into several cases of the original GM information having incorrect references to Suffix codes. The published info on sites is most often extracted from these sources and reformated to fit the site, so the same inaccuracies get restated. Listings based on investigated applications are harder to come by, some sanctioning bodies and collector clubs have collected it but few publish this info and then only from time to time.
See if I were to speculate about this blocks origins it would be -
'This was probably one of the original Pace Cars built with a special one-off version of the RPO-L48 from a '77 and then highly modified to meet the needs of the Indy officials ...
Like said - 'it's a late 70's 4" bore block and may be a 4-bolt' - "party on dude"
As always JMHO's
OR - the "A" is really a misread 'R' ...
A photo would allow more info to go on.