B= 1966 Street Hemi production line engine
H= Hemi
WT= Water Tested block (multiple random single letter hits, common)
1-21= January 21st 1966 assembly date
05= Engine assembly sequence
Maltese cross= .010 under sized crank / over sized bearings

The Grand National could have been added by anyone, IMO it was most likely done by a race shop of some kind or another, maybe even one of the big boys of the day like Cotton Ownes etc. and I'd further my guess that it was for a 66 or 67 vintage NASCAR race car. .02