Some things have been commonplace for too long. Times are changing, and some of it is for the best. There probably aren't too many people out there faced with the question of having a three- or four-speed automatic in their truck who would opt for three, but again, there might not be too many people who have thought about it. The 4L60 transmission has been around long enough to prove itself, and it is a shoe-in to replace a Turbo 350 or 400, plus it can be found in a non-electronic controlled version that'll work behind a Chevy motor in almost any old truck. Cruising down the freeway while floating the valves keeping up with traffic isn't much fun, and that fourth gear sure would be nice.
Gear Star is a company that's been around for over 20 years in the custom-built transmission business, and it is a firm believer in the 4L60 as a workhorse that'll take a floggin' and keep on joggin'. The 4L60 also has an electric fourth-gear lockup that totally eliminates any slippage in the torque converter and results in a 30-percent overdrive ratio via the lockup's mechanical connection to the motor, which will also help the tranny stay cool and lubricated for long periods on the road. All of this is a benefit of a trans that'll withstand up to 700 horsepower thanks in part to a super-streetable Yank Racing 10-inch converter that behaves like the best 12-inch converter off the line, but can take all the abuse your foot can dish out.
Gear Star transmissions are built to order by one of a few master mechanics with over 20 years under their belt; they don't have an "off the shelf" package. That same guy tears down, pulls the parts, builds, and dyno's each tranny. Their Level 3 transmissions have many of their own upgraded parts, such as output shafts five times stronger than stock thanks to the 300M hardened steel many of their parts are made from. Once this sucker is in you won't break it or worry about it. According to Gear Star's Zack Farah, each comes with a 4-month/24,000-mile warranty to back it up.
Replacing the old TH400 with Gear Star's Level 3 4L60 in a '72 Chevy C-10 was basic, but care needs to be taken while adjusting the TV cable to ensure the right change in line pressure inside the tranny to coincide with how much your foot is on the gas pedal. Your driveshaft might have to be shortened like ours was by local shop O.C. Driveline or a new one made depending on the application and if it's a one- or two-piece unit. We also opted for a new tranny crossmember from Performance Online that makes for more room between the frame-rails thanks to its simple yet effective design.
Your motor, wallet, and ears will thank you in the end. With the lower cruising rpm, just try to keep your hot foot cool. Check the source box to contact these companies for more info.
Start disconnecting everything that attaches the trans to the truck and the motor, like the shift linkage at the bell crank on the inside of the frame, while taking note how it came apart since it'll be used with the Gear Star 4L60. | 
It's a dirty job for most daily driven trucks, but someone's gotta do it, so the dust cover comes off. I can recall very few older vehicles that still retain their dust cover, but after taking this one off it proved their function 100 percent. The exterior of the truck's drivetrain was pretty nasty, with some decent oil leaks and road grime, but the dust cover kept everything under it quite clean. | 
The torque converter is held to the flex plate by three bolts that go in through the front side like so. These need to come out. You should be able to rotate the motor with a socket on the crank bolt to get them down where they can be removed. It's also a good time to pull the driveshaft. Watch for fluid from the tranny's tailshaft. |
A couple of minutes were well spent dropping the header on the passenger side to give easy access to the starter and have one less thing to bump your head on. Be careful with the starter--it's heavy--and label the wiring if you need help remembering where it goes. | 
Get out your best set of flare-nut wrenches to loosen the tranny cooler lines. I know I always preach about the importance of good wrenches, but if you've ever rounded off a fitting with an open-end wrench, you know what I mean, and if you haven't, there's a first time for everything. When the cooler lines are cracked, fluid will leak out, so have a drain pan ready. | 
Pull the dipstick tube and undo the tranny mount, bellhousing bolts, emergency brake cable, and any other little things like vacuum lines as well as the crossmember bolts so the engine and trans can be jacked up enough to knock the crossmember back to clear the tailshaft when the tranny comes down and out. |
The motor will need to be supported by a floor jack, jackstands, or hoist. I used a heavy-duty car trailer-type ratchet strap and hooked it to each framerail and had it cradle the oil pan with a board in between in case the strap moved. I was going to raise and lower the truck on the lift and needed everything to move together. | 
With some room to breathe underneath, I commenced with the cleaning and remodeling. A big hammer helped evict the greasy old tranny crossmember. | 
Since the crossmember was going to move forward for the Gear Star 4L60, we used a new one from Performance Online (P.O.L.) that gives more room underneath thanks to its sleeker-than-stock design. It even comes with new hardware. |