Embedded Lockers (EBL) is what was being mentioned. Outside of Howell, probably about the only viable option at this point.
TBI cannot adjust fueling when components are changed (like cam, exhaust, intake, compression, heads, etc) effectively when you are on the throttle, so it's almost guaranteed it isn't getting enough fuel under load now. More power means more fuel. All TBI "knows" is that under W load, with X engine, with Y gears, that the injectors should provide Z fuel when the throttle is depressed. At cruise the O2 sensor is used to adjust fuel trim, and because engine load is so low at cruise, normally the O2 can adjust fueling close enough to run "fine" there.
The timing curve needs are going to be significantly different as well. Some reading here...slightly different setup, but you may get the jist of it:
https://www.gearhead-efi.com/Fuel-I...hp?930-Tuned-L31-Vortec-with-TBI-Heads-Intake
FWIW if you run an adjustable fuel pressure regulator you can TRY to bump pressure up and see if that helps, but it's not a fix, that is a bandaid. You can also try and shim the stock regulator spring, but you can quickly bottom that out and crack the housing if not paying attention. You'd need at least a fuel pressure test kit to see if your pump will even go higher. The TBI pumps are considered good by GM if they will make at least 9PSI, at best those pumps seem to max out at 14PSI. It's not really much.
It *is* possible to destroy the engine in the current state. I'm not saying it's guaranteed, and I'm not trying to start something, but running lean under load is how you destroy engines. The knock sensor will help prevent that as best it can, but if you can *hear* knock, it's bad. The knock sensor will start retarding timing to try and stop knock before its even audible to you, so if you can hear any pinging, it's beyond what the ECM can compensate for by pulling timing. But it will pull timing, and it will absolutely cause a "sag" seen as a significant loss of power. I will state that thing is not and will not run right without tuning the ECM (really the PROM).
At this stage we are so far into the LS-swap craze almost everyone has forgotten about tuning TBI. You are either going to have to get super involved in it, and have the drive to complete it, or farm the work out to someone else like Howell.
You can probably google "max HP from TBI" or a variation of that to see if 260HP is supportable with the stock TBI housing, I expect it probably is, but it's another variable that needs investigated.
FWIW I'm running TPI/L31 longblock (factory injection, PROM tuned by myself, and headers), 33's with 3.42 gears and a manual trans, and it has no issues with accelerating. Yes, TPI is a different induction system, but same era, same limitations for the most part. I had to do a LOT of tuning to get the fueling and timing right. It needed significantly more fuel than the original TPI engine (L98) it came off of with the L31 (edit: and timing was all wrong too, as it will be in your case, since the Vortec combustion chambers are so different than most all prior to that design). The L98 (250hp) was a pretty formidable engine for that time period, especially compared to TBI (210hp), and the original tune to run a TPI 350 was nowhere close, even in fueling.
