From my experience it's just the "nature of the beast". Mine also is like that. The exhaust pipes run together directly under the tunnel also contributing to the heat. Could possibly try wrapping the exhaust with heat tape so that the heat stays in the pipes and is transferred further back. Maybe the exhaust pipes are very thin and that would cause it?
Sounds similar to the problem on our MGA's. Heat from the engine compartment and exhaust manifolds funnels down the transmission tunnel only adding to the heat (and with thin wooden floorboards on the MG's it really becomes a problem!!!).
As John mentions - about all we can do is add insulation to block the heat.