GM's 340278 Clutch Release Fork.

General Motors Part # 340278 Clutch Release Fork

The GM "278" Clutch Release Fork. I own two used
forks with 340278 stamped on them. GM dealerships
list this Fork as being superseded by the 140066235 fork.
The 1986 Parts history catalog simply says "340278 use 140066235".
Many aftermarket vendors list both part numbers on their boxes.

Top View
From bearing end tip to center of push rod pocket
measures 10 5/8". The N.O.S. "235" Fork that I
have is 1/8" shorter, which is the only difference
I can find between it and this fork. The "278" and
"235" forks are some times referred to as GM's Heavy
Duty release levers because of their beefy construction.

Part #
The two examples I own have the part number stamped
on the arm .

Bottom View
This example has a clutch return spring bracket bolted
to the bottom rail. Also notice the push rod pocket
on the right end is offset toward the bottom (road).

Side View
This Fork has a curve that would make it closer to the Z-bar.

Spring Bracket
This is a close up of the clutch return spring bracket.

Compare
This is a side by side picture comparing a "278" (bottom),
with a "632" fork (top)

Compare
This is a Picture comparing the used "278" to a
Dorman 340278/14066235 fork. (very close)

Applications Per thru 1975 Chevrolet Parts Catalog*
0.795 FORK, clutch release
73-74 All(454)
73-75 All(350)
75       Nova(262)
73-75 All(6cyl.)
73-75 All(6 cyl.)

Applications per the Thru 1975 Oldsmobile Parts Catalog Effective May,1980*
0.795 FORK
66-74 irrig.Eng.
73-74 L6 Opt. L34,M11,M15
73       All A Opt. M15,M20
75       All X,A 260 Eng.
75       L6 Eng.

Applications Per thru 1975 Pontiac Parts Catalog. Dated November 1,1974*
0.795 FORK,clutch release
75 ALL 250 M.T.

* All = all models covered by this catalog with specified engine and options.

Disclaimer:

Factory correct applications are your responsibility. GM made part and part # changes with no rhyme but a reason (no production interruptions at any cost).
Application charts are for replacement parts that were available at the time of parts book printing.
My observations come from the parts I have in my collection or other reliable sources your parts may differ.
If Factory correctness is your goal, do more research, join car clubs,
forums and buy an assembly manual if one is available for your Project Car (you wont be sorry).
This is a fun and interesting part of the hobby!