Lets do the math.
I'm a big fan of ls engines, a couple years ago I traded my 5pd, 4cyl Cherokee for an 04 gto w/m6. The Cherokee managed about 19/23 mpg, the gto with careful driving can manage a decent 19/26 mpg. Not that it matters, because the way I drive it yields much less. But I drive less than 10k miles a year so I fund the difference as "entertainment".
The swap, if well planned and executed, could provide a great improvement for your Firebird project. But, is it worth it?
To do this right we need to know how much you drive, before and after estimates of mpg, average price of gas, and what the swap really costs. I don't propose to argue the fine details of each parameter, instead I propose an example you can use to do your own calculations.
In my example, I assume you drive 10k miles a year, the bird is nailing down 15 mpg now and after the swap (using your 4spd as is, and with optimum tune, gearing, etc) you manage 20mpg (a 30% increase). I'll use the good gas at $4/gal; and expect to complete the swap for $1500 (low end estimate IMHO).
Monthly savings equals milesdriven*mpgafter/mpgbefore*price/12=10000*20/15*4/12=$55/mo
So before the swap you spent 10000/15*4=$2667/yr or $222/mo on gas.
After the swap you'll spend 10000/20*4=$2000/yr or $167/mo on gas.
222-167=55 (Notice the 30% savings in cost)
With savings of $667/yr or $55/mo, it'll take more than 27 months ($1500/55) to break even. While $55/mo isn't a fortune, it is worthy of consideration.
Of course you can change the assumptions used and get a very different result. Keep in mind the calculation is only useful if your assumptions and estimates are correct.
Now let's look at how changing the assumptions affect the outcome:
miles per year - double the miles, double the savings and cut time to recover cost in half (keep in mind though that double the miles also means double the total cost of gas).
ratio of mpg before/after - increasing this parameter is the heart of the matter, resulting in more savngs to match the improvement realized in mpg (again, keep in mind to achieve more improvement, expect to spend more on the swap)
price of gas - as prices go up your savings increase (the inverse ain't happening, don't get me started)
cost of the swap - increasing cost of the swap increases the time to recover your investment (this parameter is the most difficult to get right and the impact can be huge)
So is it worth it?
Put your own numbers in the calculation and you'll know (as long as you are honest with yourself, and not dreaming of going from 15mpg to 30mpg after a $1000 swap).
A few other factors to consider:
seat of the pants thrill an ls can provide
satisfaction from being smarter than the average bear (thx Yogi)
satisfaction of a job well done
pain and anguish of an unfinished project because of time, money, etc