I finished this repair on '89 760T recently and can provide the following tips. 740 is touted to be a little easier.

You will find a 1/4 in flex drive very handy for removing the screws (7 or 8 mm hex) at the top of the 'box' that holds the heater core. I used one designed for 1/4 in hex bits with a 1/4 square drive. There are a bunch of screws around this box.

You will find that there are several places where ducts enter a plenum that have foam seals. I replaced the deteriorated seals with adhesive backed foam weather-stripping from the hardware store.

To seal the plastic box that holds the heater core I used butyl rubber designed for installing winshields. It is larger in cross section than needed, but stretch it and it shrink down to about 1/8" in diameter. Worked great.

Tribal knowledge suggest to replace the vacuum damer units while everything is apart. Most noted for failure is the double acting unit. This one I replaced and left the other two in place. These are kind of pricey and that is why I chose to only replace the most notable. ($56.00)

Very important: Test the new core for leaks before installing. I adapted a piece of heater hose to the core, put it in a tub of water and pumped in about 20 psi. First unit leaked at one solder joint.

When putting it all back together, I decided to shorten a couple of the duct tubes about 1/4" to ease re-installation. Still plenty of engagement to hold and seal tightly.

The hardest part for me was getting the inlet and outlet tubes aligned to the rubber washer and pushed through the fire wall. They are soft metal and I didn't want to put any strain on the solder joints. This is one moment where a helper would be desireable. They could align and ease the tubes through the seal as you are pushing from below.

Check local source for heater core. I deal with a local Carquest parts store and the core was only $63.

Go for it. Prices I was quoted were in the $1,200 range.

Good luck.

Dan