The cost could be justified depending on the price of gas.
Most people say an LS swap will cost about $2,500 on 96+ vehicles. Let's say your fuel economy goes from 13mpg to 15mpg. Those are the constants. The cost of gas is the variable. Here's an example.
100,000 miles / 13mpg = 7,692 gallons of gas
100,000 miles /15mp = 6,666.6 gallons of gas
7,692-6,666.6 = 1025.4 less gallons of gas over 100,000.
If gas is $1.50 per gallon you will save $1538 over that time.
If gas is $4 per gallon you will save $4,101 over that time.
At $1.50 per gallon you would need to drive your truck over 160,000 miles to make up the $2,500 cost.
At $4 per gallon you would need to drive it just over 60,000 miles to make up the cost.
It's impossible to know the exact payoff of an LS swap going only by fuel economy. The price of the swap might be a little more, or a little less. The fuel economy increase might be a little better or a little worse. The price of gas is the major question mark. It could stay low or it could sky rocket. We have gotten to $4 per gallon in the past.
The other thing to consider is the maintenance costs your old vortec might need. You already said it's not 100% healthy. Not doing repairs on your old engine shortends the payback period.
Basically like anything change or upgrade you make to any vehicle, you need to keep it for a long time to get your money back.