The problem with using just the silicone is when it gets warm it is going to expand and get really flexible, which could cause it to kink and let the pipe come loose.
If you use copper you can solder the joints together and have a rigid non-flexible connection to the pipe that won't kink or come loose.
Keep in mind that the longer the head pipe the more backpressure it will make. This in turn will cause the motor to make less horsepower but more torque. If you make the head pipe to long it will effect both HP and torque. I am sure that there is a formula somewhere to calculate head pipe length but I have no idea where to look for it.

It looks like auto2 posted a week ago here http://www.rcuniverse.com/forum/m_31...tm.htm#3195081
He was the first one to use the ct4 mounts in a big block conversion that I know of.


rolland