Thanks to all who responded to my query.
After much research we have decided to do a 1 inch block fill using the Moroso block filler. The calculations show that it will give the needed strength without any overheating probs.
The reason for using the Moroso product is that it has better rheology, hence making it easier to pour & less chance of getting air pockets, otherwise Hardblok would have been the choice. When they are set the 2 products seem to be the same.
I will post the results after the motor is built & running