Edit 7/3/14: New best of 12.91 @ 105.0 See post #19

Finally made it to the strip with the cobbled together (not rebuilt) L31/ZZ3 I swapped into the Firebird last fall. It's a 248k motor from my 98 pickup with new rod bearings, new timing set, .015" head gaskets and a ZZ3 cam (208/221/112). TC is an Street Edge 2800. 12-yr old Hoosier 26" slicks. Edelbrock Performer intake, stock TPI manifolds & exhaust.
Despite some carb issue where it stumbles badly after the 2-3 shift, it still ran a best of 13.06 @ 103.8. 1.72 60' times.
I'll post in the carb section, but I'm guessing some kind of slosh starving the main and/or secondary jets. Maybe float level too low? Maybe not enough pump capacity to keep the float bowl filled, and then the lurch on the 2-3 shift just enough to disrupt the supply? I have just a cheapo Airtex inline pump.

not bad for the first time out, the high miles, stock exhaust, and carb issues.

Good job. Not bad @ all. Getting ready to head out to Super Chevy today. Gonna be a good day.

How did you bolt the manifolds up, vortec heads lack one necessary bolt hole? I'm trying to solve this myself with out headers.

??
The bolt holes are all in the Vortec heads to bolt up the manifolds.

Maybe the TPI manifolds don't use the missing hole, the TBI ones do for sure.

Some production vortecs do not have all the holes typically found in a normal small block Chevrolet head.

Mine are from a '98 Silverado if that helps any.

GM did some weird stuff....I have a 2001 or 2002 L31 block for my boat and the bellhousing and engine mount holes are metric tapped.

Sounds like youre running the float bowls dry
Ck levels then ck FP under load youll find your problem

That was my initial thought, and I'd be 100% certain of it if it didn't recover after that half second and pull so cleanly to the stripe. Because of that, I'm thinking Fast355 might be on to something with the airvalve adjustment.
Maybe the airvalve wasn't even open far enough with the 305 motor to make the issue manifest itself then?

I'll update here when I get a chance to mess with it.

Since most of our transmissions will not shift into OD at WOT, your highest load on the engine will be right after the 2-3 shift. What better place for the airvalve to cause issue. Remember a carb meters off velocity and if the airvalve is too far open the velocity will be too low and cause it to run lean and maybe even feel like it is running out of fuel.

You are also onto something with the 305 pulling less. The 305s mostly run looser airvalve settings than the 350s. The 350s pull more CFM for a given RPM. The 305 air valve setting on the 350 is likely causing it to be too far open when the engine recovers after the 2-3 shift.

If you are running the stock choke/air valve pulloff canister on the side of the carb, you will likely never notice an airvalve that is too loose on the throttle stab off-idle.

i agree that you should tune the air valve. i kinda remember starting off with only a 1/4 turn? idk, it's been a while.

i'd also get rid of the in-carb filter. change any other fuel filters.

could be a flow problem from the pump too? just because you have pressure doesnt always mean theres enough flow. theres a test for flow.

do you have a return style regulater for that pump? how much fuel pressure before the carb?

Also a 98 Silverado engine, so your's has the rear most holes mine is missing here?

@Spaceboy - It looks like the manifold is the problem. Due to the exit location, the last bolt hole is spaced away from the #8 port (passenger manifold shown). Finding that bolt location on the head is going to be specific to the original application and non-standard for any old engine/head you use.

The engine shown in the background is upside down, and is either #8 (passenger rear) or #1 (driver front) with standard location bolt holes.

Actually the 7 and later 8 bolt exhaust flange heads are stock and quite commen. My 97 Express built in 96 had a set of 906 castings with 8 exhaust bolts. I have a newer 2002 express engine with only 6 bolts in the 062s. The 062s I pulled off my circa 06 crate engine were 8 bolt 062s.

Update:
Back to the track today to try some things.
Starting line wasn't so good. Best 60' was a 1.78, as opposed to 1.72 three weeks ago.
Running less airvalve tension helped, but still couldn't find the happy medium between bogging at the line and the airvalve closing on the shifts and costing power.
Creeping up on those 12's though: 13.01 @ 103.6 and 13.02 @ 104.3

I've updated my post in the Carb section too.

Still, for a budget build... Thats nothing to be ashamed of.

Well, my daughter came home for a visit and showed me how to drive the car. She goes out there with 6-yr old street tires, and after a couple of passes is finnessing it off the line to the tune of 1.79 60' and runs 12.91 @ 105 !
Letting the car shift itself, instead of the manual shifting I was doing about 5200 rpm, seemed to work out just fine. And tonight was also some unusually cool air for summertime.

Sounds like it was hauling the mail pretty nicely.