D4 = 74
D = Falcon
E = Engine part
6015 = Block group part number
BA = Revision
The first two digits indicate the decade and the year;
A = 40's
B = 50's
C = 60's
D = 70's
E = 80's
F = 90's
G = 00's
The third digit designates the vehicle model line;
A = Ford full size (Galaxie)
B = Bronco (70-75)
C = Torino/Elite
D = Falcon
E = Escort
F = Pinto, Maverick, Late model Falcon
G = Early Comet (61-68), Montego, Cyclone (69-76)
H = Medium & heavy duty truck
J = Industrial
K = Edsel
L = Lincoln (58-60)
M = Mercury
O = Fairlane (62-68), Torino (69-76)
R = Capri
S = Thunderbird
T = F series or Bronco
U = Econoline van
V = Lincoln
W = Cougar
Z = Mustang
The fourth digit designates the engineering group;
A = Chassis
B = Body
E = Engine
Or if it's a service replacement part;
M = Ford Motorsport or FFRP
X = Original Ford racing program/Muscle Parts
Y = Lincoln-Mercury
Z = Ford
note that just because its a D4 block (1974), does not mean the engine was assembled and installed in a 1974 vehicle. Casting numbers were used until a revision was made that created the need for a new casting number. Generally, a 74 block might have 77 heads, 76 intake, and a 79 carburetor. In that case, you could assume that the engine was assembled and installed the year of the most recent casting number.
basically, what you've got is a standard issue 302, likely with the crappy low compression heads, dished pistons, and retarded timing set.