Get the block and head number. If it is a 1954 it will drop in without water pump modifications if it is after 55 you will have a clearance issue. You will probably need to do some changes in regards to rear bellhousing and mounts as well. Its important that it turns free if not removing the bellhousing becomes a bit more problematic. I looked close at the pictures and it looks like a 235 to me you just need to check the numbers, it is not unusual to see 261 heads on some of these engines due to cracking issues with 216 and 235 originals.
CHEVROLET CASTING NUMBERS FOR IDENTIFYING THE YEAR AND APPLICATION, CYLINDER HEADS AND BLOCKS
CYLINDER HEADS
1941-48 839401 All 216 and 235
1949 3835409 All 216 and 235
1950-53 3835517 All 216
1950-52 3835499 216
1950-52 3835909 235
1953 3701887 235
1954-55 3835913 235
1954 3635499 261 Truck
1955 3708570 216 Truck
1956 3836848 235
1953 Corvette 6 Cylinder
1954-55 3836241 Corvette 6 Cylinder
1955 3703523 Passenger, Corvette V-8 265
1956 3725306
1956 3837064 Passenger, Corvette, V-8 265
1957 3731554 Passenger, Truck V-8 283
1957 3731539 Pas w/4BC V-8 283
1947 3740997
CYLINDER BLOCKS
1941 839400 216 Passenger & Truck
1942-47 839770
1942-47 839910 216 Passenger, Truck
1942-47 3835335
1942-47 839751 235 Truck
1948-49 3835497 216 Passenger, Truck
1048-49 3835309 235 Truck
1950-51 3692703 235 Passenger Powerglide
1950-51 3835497 216 Passenger, Truck
1950-51 3692713 235 Truck
1952 3835692 235 Passenger with Powerglide, Truck
1952-53 3835849 216 Passenger, Truck
1953 3701946 235 Except Powerglide
1953 3701481 235 Powerglide
1954 3701481
1954 3835911 235 Powerglide
1954 3701481 235 Excluding Powerglide
1954-55 3702436
1954-55 3703414 261 Truck
1955 3835911
1955 3733949 235 Excluding Powerglide
1955 3836233 235 with Powerglide
1955-56 3836340 261 Truck
1953-55 3835911 235 Corvette 6-cylinder
1955 3703524 265 V-8 Passenger, Corvette
1956 3720991 265 V-8 Corvette
1957 3731548 283 V-8 with 2 BBL carburettor
1957 3867802 283 V-8 Fuel Injection
