According to my resources the casting# 3837004 is listed as for 1955-57 Chevy 235's with 4 different horsepower ratings (123, 136, 140 and 155) and notes it would have the "large WP hole" (Water Pump).
I didn't find your listed "3788307", but there is a listing for "3738307" (perhaps the number was misread?). The "3738307" is listed for 1958-59 235 with either 135 or 145 horsepower and notes specifically for a "truck".
As Kevin mentioned above, locate and post the "date casting code" info located directly above the starter location and we can better help ID these blocks. Should look similar to this info below:
CON 8 D 18 7 (CON 8 = Conveyor #, D = Month (April in this case), 18 = Day of the month (18th), 7 = Year (1957)
Hope this helps.
Dan
