Borison make sure you do a compression test on the motor. If the compression is bad youre looking at a rebuild, crate or JY engine.
If you have an obd2 tool check the distributor to make sure its zeroed out. The timing on those isnt set by turning the distributor its done by the computer. The torque app and bluetooth plugin should work although you may need the paid version.
Another thing to check is the distributor gear itself. On my old truck it had worn so the teeth were like knife edges instead of being squared off. Any of the above issues will make it feel like it has no power.
If the body and interior are solid and clean it may be worth doing the above for the right price. I saw a lot of decent gmt400 2500 454s for under $5k with less than 150kmiles when I was looking. If they would have had barn doors I would have bought one. There were several available in AZ, or NV if your close to there.