Beefier engine means beefier suspension and beefier drivetrain all of which means money.
For comparison, I had my six rebuilt (bored 0.030 over w/ mild cam and modified for full pressure oiling) for $1600. I bought a rear end from a 6 cyl Firebird for $350 and a 5 speed tranny from a 6 cyl Camaro for $350. So the drivetrain and suspension are both designed to handle the power of the engine I have.
