I have a Sprint engine in my TR7 whcih I use in autotests and the kind of "rallies" discused here.
The block is a £75 e-bay engine and the head is from another engine given to me.
Because of a failing radiator, a leaking replacement and a stuck thermostat it has been very hot on at least 6 occasions but I drove a 400 mile round trip to Knockhill and competed in the Speedfair rally all day using 6000RPM on a regular basis.
The engine is still fine and gave good power - if that's an engine that is "soft" I don't know what more anyone can expect.
See here
http://uk.youtube.com/watch?v=lwkVeXr5fvI