The underlying question is what you want out of the vehicle when you are done. Are you looking for a car show cruiser? A truck with modern handling? A low-rider? Just a modern drivetrain and/or brakes?
After that, look at the wheelbase and track width of the original and the proposed donor.
You can adjust the wheelbase by cutting the frame, but that requires skill and knowledge to be safe and has knock-on effects like changing the steering dynamics, driveshaft length, etc.
You can adjust the track width *some* by picking wheels with different backspacing, but too much change and you can get yourself into trouble.
As far an the el camino, also look at the GVW for it and the panel truck. It will have much softer suspension and sit lower to the ground. Is that what you want? If so, that could be worth it to you.
The advantage of the S10 is that it has very close to the right wheelbase and the right track width already. It is also a similar weight vehicle so the suspension dynamics work decently with no modification and can be made to work very well with off the shelf parts. That swap has been done enough times that the gotchas are pretty well known and multiple kits are available to do it with minimal skill or pre-knowledge.
If you want to do a swap to something new/different, you will be in uncharted territory and there will be a lot less ready-to-go help/knowledge for you to tap into. Is that a bad thing? Depends on your level of skill and how much effort you want to put into it. None of these are reasons not to do it, just things to be aware of so you have some idea what you are getting yourself into.
I have seen people put TF trucks on everything from a GMT800 to an S10 to a 90s Caprice. At the end of the day, the process is to get the tires in the right spot with frame/axle/wheel mods, then add body mount adapters to the right spots, then bolt up the body and hook up the steering/brakes.
It is a simple process to describe, but the devil is in the details. If this is your first major vehicle swap, I would personally recommend going with a more established solution. If you decide to go down the El Camino route, please post about it and we will happily do what we can to help you along the way.
