Out back, the rearend has to be up to handling the increased power.  For a big-block, an 8 3/4-inch rearend is the best factory piece for the job; these rearends are getting hard to find at wrecking yards, but they usually turn up at swap meets, and aftermarket axleshafts are available.  The stock suspension should also be upgraded.  For safe street handling, forget the six-cylinder stuff.  Heavy-duty factory or aftermarket components with increased spring rate are the right call.

Biscuit mounts have one problem: engine torque tied to pull them apart.  Schumacher makes this trick torque strap that bolts between the K-member and engine mounting ear, eliminating the chances of tearing out a motor mount.

Since the mounting pad bolt holes on the K-member are slotted, the engine can shift from right to left, tilting it one way or the other.  Schumacher recommends using a level to make sure the engine is installed square to the car's body.

Brakes and
Suspension
Big power requires adequate brakes.  Light-duty drums such as the  factory 9-inch units that came on six-cylinder cars are out of the  question.  This '70 340 Dart came with factory four-piston Kelsey  Hayes dicsc up front.  Once rebuilt, they will be more than up to the  job.  Drum-brake cars can be upgraded to the single-piston sliding  caliper disc brakes that came on '73-'76 A-body car by swapping the  spindles and upper control arms.

Now we see the reason for Schumacher's oil pan recommendation.  Note how the idler arm and drag link fit perfectly with the factory-notched pan.

A nice feature of the Schumacher Tri-Y is that it doesn't capture the starter.  Unlike most Mopar headers, the starter slipped easily in position, and bolted in with the headers in place.  It won't swing in with the usual lower mounting stud in place though; use a bolt instead.