First, find out several things about your present driveline, and the type used by GM. Need to compare overall length of bellhousing to rear of C4, then OAL of the T56 bell to it's rear, including the transmission. If different, new driveshaft is needed. Also, type of front U-joint connections used: common Cardan joint, or a round flange with bolts, like many Fords use. Resolving that, if you go ahead:

Then the clutch linkage/throwout bearing system must be done. Maybe use early Mustang mechanical linkage, maybe later cable-type easier to fit up? Clutch swing pedal has to be added under the dash. Which type depends on linkage type. Or, last resort, a "juice" clutch release using a master and slave cylinders like on Ford F-150s or Explorers, around 1995, '96. Either need a clutch pedal for them.

Gotta get a flywheel to fit 429 big block, make certain of the proper balance-weight on it. Clutch type used must be compatible with the flywheel. Finally, if you still have automatic-transmission rear gear ratio, very low numerically, like 2.69, unless the manual trans. has a quite low first gear ratio (that is, high numerically, like 3 to one first gear), it will be very "boggy" from a standstill.

How much work of this nature have you done? Are you up to it?

 

 

Personally, I dislike the hydraulic clutch release for a couple of reasons. First, and most important, they feel kind of "squishy" compared to a solidly-made mechanical link-up. Second, the slave cylinders in most cases are mounted INSIDE of the bellhousing, which means there is no throw-out bearing fork, which was used when the slave was mounted outside the bell. The slave starts to leak inside the bell, ya got a messy job, difficult, remove transmission to repair. If I remember right, the t-56s I've seen had a partial bell already cast on the case- very heavy to handle.
Cable-release system, like Fox bodies used is pretty foolproof, but still inferior to mechanical. The cable rolls-up on a semi-circle quadrant under the dash. Stock quads are plastic; first thing to replace with an aftermarket metal one. Since cables can only PULL, not push, the throwout fork has to work kind of backwards, with it's throw-out pushing the bearing in the CENTER of the fork, the pivot at far end from the cable-attach. Mechanical release PUSHES the fork, the pivot is between the outside end and the throwout end.