Since dragging Bobs out of the woodwork has become a theme here...................
If time and money are the primary motivations/obstacles, all joking aside, the sbc/auto combo is a slam dunk decision, even if you own a 351 core already. They fit the '40 like a glove; no firewall mods needed and a mechanical fan will still fit too. No funky front cross member mods or center X member either (simple bolt in kits for trans mount available). Sure, the sbf can be made to fit similarly after oil pan and pump change, and water pump change, and a couple other little things, but by the time you do that you've covered about half the cost of a rebuild on the sbc. All the whining about the sbc being a "belly button" deal misses the point of the pile of good reasons that made it that way. It saves time and money (which are really the same thing), and it works great. All the other choices are fine, but take more resource.........ya makes yer choices..