Been thinking of putting a heater in the back of the car running the water pipes down the trans tunnel.

ATM the highest point in the system is the expansion tank on the front of the scuttle.

Would the heater work if it was about the same height or a bit higher than the tank or am I simply asking for trouble??

All the best,
Steve

Usually you want the expansion tank to be the highest point in the system...

Having the heater matrix higher would for one thing make bleeding it a bit of a PITA.....

Pretty well all rear heaters are a pain to bleed. Your best bet is to mount it low, and tilt the core so that any air will come out into the hose. You probably will still want a bleeder nipple down there somewhere though.

Where were you planning on having your air inlet?

my heater is under the scuttle on the passenger's side. The hot air comes through a pipe with an outlet pointing at my shins. The effect is just fine.

The heater matrix is just as high as the expansion tank, which is a little higher than the engine and radiator. Draining the coolant from the engine leaves water in the heater but that's no problem as it stops air getting locked in there. I fitted a bleed valve on the return from the heater to the rad which makes getting air out easy. Because the feed to the heater is before the thermostat, all the water flow from the cold engine goes through the heater, flushing out enough trapped air. Actually, the system self bleeds fairly well.

You could probably do it if you fit a bleed nipple next to the matrix and also when you bleed it make sure to find a steep hill and park it facing upwards so that the matrix is lower than the rest of the system. Doing this though could make it difficult to bleed the rad so you may decide to fit a bleed nipple there to.

I use a trick on some cars which is to fill the systems through the nipple rather than the tank. Works sometimes, not always.