I watched Dennis Olthoff race the GT40R last fall at Watkins Glen at the SVRA races, and am sure that he is running a H-M small block.
He ran VERY well - running right with the Group 7 cars with much more displacement.
Give Dennis a call. I am sure he can give you some real world feedback.
http://www.olthoffracing.com/
704-647-9924
hope this helps,
Jim Kellogg