I will echo the above.
You don't really want to swap a modern engine to your older transmission either. First, there is no other engine that will bolt up to the FE-pattern transmission. So, at best, you'll have to find a different version of the transmission to bolt to the engine of choice. If you have a C6, they made small-block pattern C6 for later model trucks (and four years behind 351C), so you could bolt in a late-model 302/5.0 with fuel injection. You might make more horsepower, but you'll be down on torque. You will lose a lot of weight off the front end, so handling would be improved if you matched springs/shocks to the engine weight. You'll have to adapt your accessories to the new engine, or new engine accessories to your chassis.
Engine swaps have a way of getting complicated and affecting other things...
A fresh 390 is more likely the way to go. They're reasonably inexpensive (for an FE) and readily available (unlike many other FE) and
parts are still pretty easily obtainable. It will bolt in and all your existing accessories will bolt right up.
If you're truly interested in doing something like this - I highly recommend getting the entire drivetrain and electronics out of a single donor vehicle and using a comprehensive
shop manual with very good and detailed
wiring diagrams to ensure it's all hooked up correctly. If you've never done something like this, I highly suggest giving it to someone who has, or at the very least, enlisting their help.