I have to agree with G. The vortec heads are better than stock 70's heads, but its not much.
Back in the day I had a Firebird with a 355 flat top motor, .488 / .510 cam, Weiand dual plane intake, 750 dp. It ran 9.0's in the 1/8 with 4.10 gears, powerglide tranny with 3200 stall converter. I switched to the vortec heads and it only ran a best of 8.93. I thought they would help it a bunch, but it was less than 20 hp I would guess.
That sounds like plenty of motor to have some fun with in your truck. My old 355 ended up in my 67 pickup. It does some good burnouts, but I have never taken it to the track.