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We’re recycling wherever possible. Our old K20 project donated a TH400 backed by an Offroad Design Doubler, a transfer-case setup that combines an NP203 and an NP205 to produce a 4.00:1 low-range. Swapping this heavier-duty drivetrain into a regular-cab Stepside requires some finesse.

We’re recycling wherever possible. Our old K20 project donated a TH400 backed by an Offroad Design Doubler, a transfer-case setup that combines an NP203 and an NP205 to produce a 4.00:1 low-range. Swapping this heavier-duty drivetrain into a regular-cab Stepside requires some finesse.

The K10’s existing TCI Truckmaster TH350 (foreground) was trail-prepped with a 2.75:1 First gear and an extra-capacity oil pan. It originally had a Doubler, like the TH400 that’s replacing it (background).

The TH400 bolts directly to the RamJet 350, but this transmission is bigger than the TH350 it replaces. Modifications are necessary to achieve the optimal driveline angle.

CRC Performance Transmissions did several test-fittings. Clearances for the transmission and Doubler with its optional three-stick linkage were checked and rechecked.

The goal is to have the rear output yoke point about 3 degrees downward. This can mean raising the transmission’s bellhousing end higher, adding a body lift, or shimming down the crossmember.

Lowering the crossmember is easier, but maximizing ground clearance is preferable. CRC modified the floorpan to get the TH400/Doubler’s nose higher in the hole. Here, the tunnel is bent up using a jack and a block of wood.

CRC owner Craig Calkins sectioned the floorpan to clear the Doubler’s linkage and to allow the new geartrain to be mounted without lowering the stock crossmember below the frame. The truck’s full cage and integral seat mount limited the sheetmetal’s upward mobility.

The optional three-stick shift linkage enters the cab farther back than it did behind the TH350. CRC was able to use the factory pivot points and get the sticks to clear the dashboard and rollcage by setting them to their shortest throws. We’ll patch the tunnel during the interior part of the revival.

The stock crossmember was reused but repositioned on the frame. We’ll measure for new driveshafts once the suspension and tires are in place.