[Dwight]

A huge different in price.

The 390 has a 4.05" bore and a 3.78" stroke. good low end torque motor.

The 427 has a 4.23" bore and a 3.78" stroke. Great high rpm motor.

The first 427 had 390 truck motor crankshaft but they broke very quickly. Ford made better crank and it broke, then a much better crank till 1965 they had a crank that would last 24 hours at 7500 rpm.

The blocks were improved every year till they could last 24 hours.

The 390 is a great motor for a street Cobra. But the 427 is the holy grail of Cobra motors.

The big different's is the price.
A 390 block may cost $100 but the 427 would be closer to $2000.
And the crank and pistons cost a lot more in the 427.

Remember most 427s had the hell beat out of them and a lot have sleeves or other repairs.
Make sure you get what you pay for.

Dwight