Hi,
I'm thinking of putting a gt exhaust on my 1600 tp and wondered if i can still run a heater box-winter can be cold in Melbourne.

gt exhaust? photo?

The answer is yes, but the question is why

Most of the GT style exhaust are no better or worse than stock.

if you have a stock motor with a single carb, you loose the push pull effect of the proper heat risers when you put an aftermarket exhaust on. Yes yes - there are exceptions but they are few and far between
Without this you will get icing of your manifold in winter

Personally, you would be better off getting a stock exhaust and get a flange hit put on and really enjoy the warmth in winter
If you have spare money get your heater boxes and exhaust ceramic coated

looking at this

they are ok - but i would put a flange kit on

The biggest problem is they have the heat riser take off at the same pressure point

if you look at a stock exhaust the LHS comes from near the collector

Go buy a vintage speed exhaust, heat exchangers will bolt up etc.

You don't have to use the J pipes with that exhaust. As far as I know it will connect up fine to your stock heater boxes. Agree that the heat risers won't work all that well, so you may get icing in Melbourne's winter.Others on here with more knowledge will be able to comment however I think if you use a gasket with a smaller hole on one side of the heat riser and a full size hole on the other side,this will promote flow from one side to the other. From memory, these gaskets are available
Also depends on the rest of your setup as well.If you have a working thermostat and stock air cleaner with with a hot air intake at startup, you may get away with it.
Cheers......wayne