I made my own mounts for my 455 into my S-10 frame. It worked out much better. For street use stock manifolds are better than headers anyways. Oil filter relocation was not necessary for me, I use a slightly smaller filter (the same as what any new GM pickup uses), and made a slight notch in my frame for clearance. Again, different frame that you're using.
When you put an aluminum intake on, the 455 is about the same weight as a SBC.
Keep your original radiator or use one made to fit your truck, I wouldn't go with the Buick radiator in your case, unless it fits really well.
I have an Edelbrock performer, Comp 268H cam, etc. in mine and it works great. A nice bumpy idle and it just flat works with loads of torque.
