94 GMC Sierra Parts On An 89 Crew Cab

[hatzie]

1) It's not likely to be a DROP IN deal...
Likely you could use the 94 column... It is a Saginaw column just like the 1989. Folks mount Caddy tilt/telescoping columns in other cars and trucks... not just GM vehicles either.
Carefully pull em both out and lay em both side by side where you can easily look em over.
Are they the same or close in length? 1/2" - 1" is not the end of the world. The distance that's most critical is the wheel to the under mount. Slightly shorter or longer through the firewall isn't hard to deal with... Think XJ Jeep U-Joint steering shaft... Google it.
The firewall plate can be swapped as needed. The hot rod boys do it all the time.
You'll likely find the under mount is in an OK position or can be moved without a major hassle.
The steering wheels are interchangeable between the squares and the 88-94 T400 series columns pick the one you like and run it.

Wiring is the one that scares most folks... OOOGIE BOOGIE I'm so afeard... Do some planning on paper. Get some Delphi or SIR Tools terminal extractors and print out the wiring diagram pages to take notes on in pencil. The nice thing about the 11x17 Tabloid PDF pages is you can have the printer at FedEx Office scale em up to 12x18 ARCH-B. It's surprising what a difference 1" in both directions makes.
I'd be willing to bet GM used the same connector families if not the same connectors on both columns. Worst case you can adapt the 94 column pigtails to mate with the 89 wiring by swapping connectors on the column or cab harness as needed.
The 94 T400 Backup/Neutral-safety switch should plug right into the 89 RV series wiring.
The ignition switch probably is the same. A cursory glance at the wiring diagrams tells me that both the 89 RV and the 94 CK ignition switches mate with Delphi 12010966 & Delphi 6294641 plugs on the cab harness side. Look over the wiring diagrams and make sure of the pin-out before you plug it in.
The turn/dimmer/cruise/wiper stalk connectors may need to be re-pinned or pinned into a new connector. I posted the 88-93 GM T400 CK wiring diagrams so dig into em and see.

2) The 1994 T400 still uses the Harrison R-4 Radial compressor. You can mate this with your squarebody condenser and evaporator setup. I'd keep the hoses etc from the T400 chassis, on the engine bay side of the firewall, including the condenser. I used the Stock Sanden HT6 from a 1998 Diesel in my 1976. I had to make some custom hoses and added the wiring for the HPCO switch inline between the LPCO and the compressor clutch so I could run R134a if I decided to use R134a.