You'll need to modify the T-bar or get a TH400 one.
New speedo cable and new hole in the firewall for it.
New gearbox crossmember.
New tailshaft and yoke for TH400.
Cable kick down setup for TH350.
Kick down switch for TH400 if not still there.

I don't know of any such place. If your car was Trimatic before it should already have a kickdown switch and wiring, just need to modify it to get to the TH400 switch. Same for the vacuum attachment point for the vacuum modulator, use the vacuum take-off tree off the old 253 and use it on the 5 litre.

Terry, I took it as meaning Andy has a Dellow Chev to Aussie box bellhousing as he states it is a 304 (which I initially thought must be talking about a VK-VL 304 and hence a manual block) but the mention it now has a carby leads me to thinking it is a VN-VSIII engine hence TH pattern block, hence the assumption about the bellhousing.

Andy, Terry is spot on with the mods to the T-bar. Hopefully you still have the wring harness for it - it will plug straight into the HZ harness, just remove the bridging plug in the detent circuit that a manual has, and disconnect/remove the reverse switch harness from the 4spd and plug the T-bar harness in there too. The grey wire will plug into the loom near to the radio somewhere and there is a black with ring lug that you attach to the body.

For the crossmember see if you can find a HQ-HZ sedan/coupe/wagon/Statesman with factory TH400 and grab the crossmember. The same car if it is a coupe or sedan will give you the tailshaft too for the TH400, if yours is a wagon the you want a TH400 tailshaft from a TH400 wagon, Statesman, ute or van.

The kickdown switch for trimtic/TH400 is up high on the accelerator pedal - it is a slide looking thing. There would have been a wire running down to the back of the trimatic from the engine bay for it, on a HZ should have also been another one for the pollution switch.

If you go for a TH350 you need a late HZ or early WB 308 gearbox crossmember from a wagon, sedan or Statesman, plus the kickdown cable and kickdown bracket attachment off the engine. You can buy generic ones and adapt it to a Quadrajet pretty easily.

Both TH350 and TH400 need the engine vacuum pipe connection, do as I stated in an earlier post.

The other problem you might face is either box may foul your exhaust.

Speedo cable, look for a HQ-WB with TH400 or TH350 (for WB has to be one without cruise control) and see where the speedo cable exists the firewall (towards the centre). On your car there will be a dimple there to show you where to drill. Remove the engine and use a (from memory - check first) 32mm holesaw to make the hole. Use a blank speedo grommet in the original hole. Factory cars also have a support clip off the closest heater box bolt, and also another screw through the passenger floor with another support clip. Look at a HJ-HZ parts catalogue and you'll see what I mean.

I have seen both Dellow and CRS Chev to V8 4spd housings. The CRS ones were multifit at one stage for both Chev or Holden pattern.