You should be getting at least 10 MPG unless you have another 6000 lb of lead in the bed -you would be getting about 9mpg. That being said there is not much as far as as smog related stuff on your 454.
First things first - TBI will you get you better fuel consumption then Carb under normal driving conditions. One of the possible reason why you are burning so much fuel is a broken or disconnected CTS. Some diagnostics are required. Luckily early TBI systems use SES light to indicate trouble codes without use of expensive scanner. All is required is an unfolded paper clip inserted into ALDL connector between terminals A & B to force ECM into diagnostic mode. Suggest that you download the following reference document:
http://www.73-87chevytrucks.com/tech...TBISystems.pdf
On Page 13 of the pdf file ALDL plug and codes are described on subsequent pages.
//RF
__________________
"The Beast"
1975 Chevrolet C20 longbed
350/
700R4! with 3inch body lift
Dual Flowmasters Super
40's!
TBI retrofit completed (2007-07-29)

New 383CID (+030) 08-304-8 9.5:1CR x36,005 (2012-12-17)