You can bolt a V8 to your existing mounts but the motor will be close to the firewall and you will need a fan shroud,the sixes have three parts to the monts and just remove the part that mounts the cushion to the block and the motor will bolt right in but if your truck came with a manual trans you will need a automatic crossmember. I done this to a 67 C20 25 years ago and kept the 4 speed with granny trans it came with.