5. Secure Sealing
For stock or mild engines, that $29 gasket special may do the job, but blowing a head gasket or sucking or spitting oil out on the open road will immediately spoil a good highway cruise. Premium gaskets from sources such as Fel-Pro are worth the added price, as the hemorrhaged gaskets shown here woefully attest. When cylinder pressure gets more serious from compression, nitrous, or blowers, think about moving up from OE replacement-grade head gaskets to wire-reinforced premium head gaskets from Mopar Performance or Fel-Pro. O-rings are probably a good idea if you're heading for the twilight zone in horsepower generation.

Drivetrain Durability
6. Ratio Realism
No doubt for the digs, you either go low with the rear gears or go slow at the green light. On the open highway, though, those 4.56s will leave you eating parts, literally winding up to ruin. The cruiser concept is to build low-end torque up front and gear-up in back; on the open road, trashing those 3.23s for 4.10-plus killer cogs is the wrong move.

7. Freshen It
Mopar drivetrains and trannys were the toughest in the industry, but for street survival 30 or more years later, it pays to start fresh. When rebuilding an old Mopar, it's tempting to just clean up major components, bolt them in, and hit the road. However, even if major components, such as the transmission or rear, are functional, they may be about ready to fold, depending on the mileage or the abuse. To keep them in the game often, all that's required are new bearings, races, seals, and gaskets, with the other hard parts simply cleaned, inspected, and usually reused. Here, we have a four-speed kit from Brewer's ready to go in. Some new bearings now can save lunching a hard-to-find or expensive gear cluster later. Same with syncros; step up with fresh brass and save those dog teeth on the main-drive gears. We swapped a junkyard overdrive 833 in our Dart, only to find the first-gear syncro was boned out. Lesson: It pays to go through it first rather than do the job twice or find yourself stranded.

8. Overdrive Options
For highway starring, stepping up the gear (lower numerically) is a good idea, but for some, giving up that righteous off-the-line dig is unacceptable. While all of the old Mopar musclecars were 1:1 at the tranny, swapping to an overdrive can open the best of both worlds. Automatic cars can use the Mopar 518 overdrive or keep the 727 by adding a Gear Vendors overdrive. Stick machines can choose from low-buck factory 833 OD trannys for an easy bolt-in swap or move to an aftermarket trans, such as the high-end Richmond six-speed pictured above.

We like the flexibility of the OD option, but don't forget, the driveshaft speed is still dictated by the ring-and-pinion. The wide-ratio factory four-speed overdrive has an extra-low 3.09:1 first cog, with a sprawling leap between each speed, and it needs mods to live under serious torque. Though automatic overdrives or aftermarket gearboxes take fabrication to install, the OD may be the best modification a road-going Mopar can get, depending upon your intended use.