i was wonderin if a 350 out of a 76 K5 blazer would bolt up to a 700r4 transmission in a 88 silverado half ton 2wd....i need to get rid of the smog stuff because the smog 350 in it now only gets 9 miles to the gallon...would i have to get a whole new wiring harness if i go from EFI to carberated because of the computer ???  

First it will bolt in. Second if you are getting 9 MPG it is not because of smog because the 350TBI is more fuel efficient than a 76 smog motor which has even lower compression and lower efficiency. You need to be looking at tires, axle ratio's and engine mods that are causing MPG to puke, not going to a engine that will likely get even lower MPG than you are getting now. I am VERY fond of 350 TBI motor and have one now 19 years old with 180K plus miles on it that I bought new and it still runs great and gets decent MPG. That engine responds well to advancing timing to 8 BTDC and running 89 or better octane fuel in it. BTW, I do not run 87 in ANY of my 87 on vehicles and newer which have higher CR in them than older ones and it is far cheaper to drive them on better fuel and they run better too.  

i have the 76 350 4bbl in my mud truck with 35x12.50 tires on it and it gets better gas milage than the 88 350 EFI...everything on the 88 is stock....i had it tuned up but besides that it is completely stock...my grandfather said when he bought it new in 88 it was getting 21 mpg..what should i do ??  

Stock tires too??? If so you might start by changing ECT sensor (engine temp next to Tstat housing) When it goes bad it can cause it to go over rich. Also that engine has 9.1 to 1 CR and it really likes 89 or better octane, especailly in warmer weather.  

i did a computer analysis on it and the only thing it says is that it nees an o2 sensor...yes it has stock tires on it too  

Code check will not show bad ECT sensor unless it is missing. As they go bad with age they make ECM think engine is colder than it is and enriches engine mixture.It is wise to change them in a older engine. O2 sensor will help too.  

where are they located on the engine and how many are there ?  

There are two engine coolant sensors but one next to Thermostat housing is the one you want. Should be one O2 sensor on driverside exhaust  

i changed the temp sensor and added a can of really potent fuel injector cleaner.....i cant change the o2 sensor yet cause its too hot under the hood....ive been runnin it all morning...why did you say it ran better on a higher octane fuel ?? what does it have that makes it run better like that ?  

would a throttlebody spacer help with fuel milage ?....i was thinking of buying one...but i dknt know yet  

Many do not understand newer engine octane needs. FI GM engines have much higher compression than old smog carb ones or about same CR as 1972 and earlier truck V8's. Back then they need more than 87 octane too but they did not have the ability to mask it with spark/knock control that detects early signs of knock and retards spark to quell it BEFORE you here it. It keeps you in ignorant bliss because you think all is well but it is also killing power and MPG while doing it. As a engine gets older it builds up carbon in cylinder chambers which raises CR even more over time and further increasing octane needs. BTW the octane of a fuel determines the temperature and pressure at which is will self ignite or also detonate once a flame front starts. It also has a small effect on the speed of the burn as well to further inhibit knock. The higher the octane fuel the more resistance to knock and the higher the CR. the more energy you extract from burn and more power and MPG too. The "trick" is to start burn early enough in cycle so that peak pressure are reached at optimum piston/rod/crankshaft angles for best efficiency. Higher octane fuel in a higher CR engine lets this happen. With lower octane fuel ECM retards spark masking knock and power and MPG can take a dump. The ONLY reason that there is a knock sensor on modern GM engine is to limit consumer complaints of motor knock as they strive to burn cheapest fuel they can find thinking they are saving money. (BTW all EPA MPG test have been done with 93 octane fuel for about 30 years now but they are not required to put that on sticker) The 350 TBI responds really well to 8 BTDC static timing and better octane fuel and power and MPG will go up noticeably and be cheaper to drive overall.Many think it says 87 in manual so that is all I need but it say minimum 87 as that is the lowest fuel it will "tolerate" not what it will do its best on. To place a 89 over better octane requirement in manual would be a sales killer because many have mind set that they save money using cheapest gas possible. Oil companies love it because it is cheaper to make and you use more of it without knowing better and they make more money. A can remember when gas was 89, 94 and 100 octane at pump and 72 and earlier engines were beasts too. Unless you lived with a pre 73 smog motor on good fuel, you never will know what you missed because a 76 smog motor is not the "gold" standard here and is seriously smogged by design.  

i saw where you were talking about knock sensors....my trucks knock sensor is broke off...the wires are disconected...will that hurt anything or should i replace it ?  

I would replace it. When it is missing I think it retards default curve when lacking input signal. The version of knock sensor (generation 2) basically replaces vacuum advance with a 20 degree range electronic one that can removes its electronic advance when knock data is present.