weewilly,
The original part number for that bolt is GM# 3696542 (Parts Group 4.661).
Dimensions specifications from the Parts Books indicate:
Overall Length = 1-11/32"
1/2" x 20 external thread end (fits into the master cylinder end cap through the Block)
7/16" x 24 internal thread (hole in end of bolt head with inverted flare at bottom for brake line connection)
3/4" hex head (bolt head outer dimensions)
I haven't been able to find anyone that currently carries or remanufactures that "banjo" bolt.
Your best bet if at all possible is to try and salvage the bolt for reuse.
That Part# is listed for use on 1951-54 Passenger cars (most with manual brakes), 1951-54 1/2 Ton trucks and 1955-57 3A series (1/2 Ton) trucks with manual brakes and standard transmissions. These would be potential donor vehicles if you can't salvage yours.
Dan
