Is it possible to change a 425 to 455 ?
Mine is a High Compression 425 (1965) so stock output is 310 bhp.
What do I need to replace to get it up to the 390 bhp like the Toro?
I've already done the intake and 4 bbl carb upgrade.

 

It sounds like you have been bitten by the "HP bug". You have a high compression 425 with a 4bbl and still want more. The HP bug will cause you to make poor decisions, cause the failure of other drive line parts and empty your wallet faster than you can whistle dixie. I suppose you UK guys whistle some other tune but I hope you understand where I'm going with this.
You have made the best upgrade already with the addition of the intake manifold and from here things will get very spendy. The intake should have been very noticable, now your talking camshaft changes, rear end gears, exhaust upgrades, head work etc. It never ends.
I would try to be content with what you have.
I spent thousands last year on a motor that blew up in 15 minutes and the builder didn't stand behind it. I look at things a little differently now.
Good luck,
Dave

 

Dan,

It's certainly possible, since the two motors are externally identical. I doubt, however, that you would find a significant HP difference. Your 425, with the correct cam and big valves, will make nearly the same HP as a 455.

 

Was just curious really.
I heard from someone that a 455 is the same block.

I think i'll stick with what i've got

 

The block castings are actually different, but interchangeable. You can swap the crank/rods/pistons from one into the block of the other.

 

A cam and headers will give you 400 HP at the flywheel with your 425 easily. Beware if interchanging the blocks, because the 455 has reliefs cast in for rod nut clearance, and longer bore skirts to support the piston skirts of the 455. You need to get the cam to match your lifter angle; a 65 block should be 45 degree and later blocks are 39 degree. You can stick tight fitting dowels or tubes into a right and nearby left lifter bore and see if the included angle is 90 degrees or noticeably less (12 degrees less).