I agree the 3550 will do just fine.
Lainhart runs a 3550 behind his 65 side oiler with 620 hp / 630 tq for years.
Hodges has a 408 with 530 hp / 540 tq with 11,000 miles on it.
Both shift very easy and smooth.
The 427W uses a 351W block so the outside of the motor will look the same. You could use the stock
oil pan.
Dwight