3500-3700lbs even seems a little light to me for a 69 Chevelle, unless it has been on a diet (fiberglass, etc..). A Camaro weighs in the 3500-3700lb range in stock form (talking 1st and 2nd gens). Don't give up hope it will still be a fun car and there is always nitrous, LOL.
The new LS1's are hard to beat they have a different valve angle and are very efficient, can make big power. Though the 388 will feel a lot different, will make lots of torque.