Hi.
The Rossa was designed to use a Classic Mini heater, preferably a metal one like this
http://www.ebay.co.uk/itm/Classic-Mini- ... 589b7d4aba rather than a plastic cased one, and you can see it in place here (this is a 'before' photo.......)
Sorry it's not a very good pic but it's the only one I have. Fitting is straightforward and the pipes do indeed go down the tunnel. I actually used lengths of copper water pipe for the tunnel but rubber is probably easier (less joins) and won't rattle....
The next pic shows the engine plumbing but again it's not very clear. However, the hose coming out of the back of the thermostat housing sandwich plate goes to the heater and the one that you can just see coming out of the bottom hose at the left hand suspension cone housing is the return. This is an A+ engine which doesn't have a heater valve on the cylinder head - if yours is not an A+ it won't have the thermostat sandwich plate, the valve on the head being used instead. The bottom hose is Metro and the bit going round the back of the exhaust towards the clutch housing is the one that goes to the header tank. It's all fairly self explanatory and your biggest problem might be getting a heater in good condition.
You can get a build manual from Alan Britton at Rossabitz 01733 844839. Hope this helps - good luck
Raymond.