Installing a 425 into my 87 cutlass

Dec 23, 2018
Hi, I’m in the process of installing a 67 425 into my cutlass. I’m using a 3:42 grand national rear, and a B&M th 350. I’m on the fence if I want to remove the AC or not. Also, do I just use 307 front springs? I am probably going with the stainless headers for it. Any input here would be appreciated. Thanks!

C716B72F-2D0C-417D-88B1-B45CAE6F751A.jpeg 91E3A13F-109A-4A8F-B070-FF4FBA451CBC.jpeg 6D13EE4E-C4ED-4D3D-A5F5-909A15326D40.jpeg
Oct 3, 2018
Braceville Ohio
Without the A.C. compressor and using an aluminium intake the "big block" (actually just a taller block) will be about the same as a 307. So the springs should work. As for the headers, I know Hooker made Super Comps for the 400/425/455 Olds. The drivers side tuve came outside the frame rail. I think Rocket Racing and Dick Miller had inside the frame rails for the Olds powered G-body but grease up your rearend when you see the price. If your headers are the cheap ebay headers, you might be in for a surprise when you try to install them. Rearend sounds good, you might want to "upgrade" your driveshaft and U-jionts. I use a local truck repair shop to make me heavy duty driveshaft. Good luck and tell us more about the brand of headers and their fitment. Good luck
Oct 8, 2018
Kokomo Indiana
I have had many 455 powered G body cars. As long as your removing a 307, there is very little difference in weight. If you use a aluminum manifold on the 425, I bet it’s about the same weight. The only times I messed with swapping springs was if the car had a 3.8 v6
the 425 block weighs about 100 pounds more than a 307, the crank is considerably heavier, and an aluminum intake is a good idea, though a tall intake will probably eat the hood. BBO in a g-body will be tight with the a/c box in it. you may need notched valve covers to keep the a/c box. you may wish to reinforce the frame for the torque.