Ok, first question as things come together.
What's the difference between a 4L60 and 4L80? Just strength?
Strength, parasitic drag (little more on the 4l80e), and the 80 is a slight bit bigger.
This truck is a pavement pounder that might see a boat or utility trailer from time to time. So if I come across a 5.3/4l60 combo in good shape with throttle cable operation, I'll pounce on it. Same goes for a 6.0. I'm not really leaning towards one over the other, it's just a nice daily driver. However..... if I come across a 6.0/4l80 in good shape......
Think about stock applications for each. The 5.3/4l60e came stock in a 1500 pcikup with a tw rating around 8000lbs or so. It will get a couple MPG better than the 6.0. In a 1/2 ton cruisier truck, i would just get a 5.3/4l60e. Plenty strong, plenty fast, and a couple more mpg.
Wondering if the larger trans is going to give me floor clearance issues or not. LOL and on this project, a body lift is out of the question.
I really need to start researching some other similar builds. I'm trying to make this all happen as quick as possible. I only have parking for two cars and the city charges $150/year to park on the street. (if they feel you don't
need a spot, they can reject you) Also, they are not going to like seeing this build out in the open, they always give me grief. Once the C10 is reliable, my S10 will be for sale.
I dont think there is a big enough external difference between the two to worry about.
My biggest worries right now are:
-Proper tank under the bed with proper "in tank fuel pump" set up (can I run a blazer tank like the '91 in my K5 now?)
Thats what i would do.
-Shifter linkage. Also trying to find which older Chevy van people are pulling the tilt steering columns from. I guess they are a direct replacement for the older C10s. Aftermarket companies want $$$$$$$$ for columns.
Pretty sure its any van from like the early 80s to the late 90s.
-Wiring. I don't like the job I did on my K5. Since removing all the electric window stuff, I can simplify my wiring once I remove my dash in the K5. For the C10 however, it's all manual windows and will more than likely get an aftermarket HVAC system to clean up the firewall. So I'm trying NOT TO use a regular fuse panel, but something easier to work on and modify. Just don't know what or how.
PERSONALLY, I would get a new wiring harness for the 67. And then have your LS harness modified by the modifer of your choice, unless your feeling a little extra frisky, then a new LS harness would be the ticket. The new 67 harness would only have one or two wires to be removed, and the ls harness would only need 3 or 4 wires to be hooked up. Easy money.
My head is just full of crap right now. The frame, suspension, and driveline will be the first to get done.
Needless to say, I don't need much more for the chassis. It will be a 3/5 drop. The 4/6 drop is nice looking, but not practical for my areas or a daily driver. Also, some of the parts are specific to LS/4L60 swaps 
