Welcome to CHR Kooksville. I don't have any experience in doing what you want but there are alot of people on this site that will give you many different answers. Sounds like a great street rod your going to be building especially with the straight 6. I would go with the ford 300 as you can do alot with that engine as you have alot of thickness in the cylinders walls and that engine has a hell of alot of torque. I had two of them in a 60's ford truck and with headers and running a small 4 barrel it's like a V-8. Good luck with the project.