Jim, you can run a 4.3 TBI with either 3.4 or 2.8. A stock 2.8 might be a bit rich with 4.3 injectors so usually you'd put the 2.8 injectors in the 4.3 TBI.

For a 3.4 application, you need more fuel so you'd use the 4.3 TBI as is.

If you were to stick with a stock 2.8 TBI on a 3.4, you'd get lean surging on warmup. Installing 4.3 injectors in the 2.8 TBI's injector pod will cure the lean surge but you're limiting the midrange and top end of the 3.4 'cause it wants/needs more air/fuel.

I used a Holley Big-Bore TBI (roughly the equivalent of a stock GM 4.3 TBI) on my old 2.8. That motor was hot-rodded with larger exhaust, bored intake, jacked-up timing, flipped lid/K&N, and 1.6:1 roller-tipped rocker arms. It ran quite well with the Holley but was just a wee bit "over carbureted".

Putting a larger TBI on an otherwise stock 2.8 would be overkill, since unmodified this engine just can't flow enough air to achieve significant HP gains.

Once uncorked at both ends, using Good Old-Fashioned hot-rodding techniques, it'll really wake up.

Anyway, hope this clears it up a bit.........ed

P.S. Other Jim, Good Job! on the swap. Now get'er run-in, tweaked-out, and enjoy the ride! Your ECU will take a while to adjust to the new motor, and you can pull trouble codes out of your auto tranny's computer to see what's going on there.