1990 350 spec's suburban 4x4

Can anyone tell me what the difference between a 90 350and a pre-86 350? I have a couple of built 70 era 350 that I have built in the garage.BUT I haven't had the oppurtunity to open up this 90 350. Does the 90 350 have a roller cam? What are the difference in the heads? This engige has TBI. and I'm an old CArb guy. This thing only gets 10mpg in town. I had a 70 chevy p/u that I could get 12- 14mpg out of it w/ a carter carb and my foot in it. What can I do to get better milage? w/ 3.73's and 235/75/r15 tires and 700r4 tranny.  

In a nutshell, the TBI350 and carb 350 are pretty different in head porting and CR ratio too. TBI heads swirl mixture in chamber which increases low and midrange response and torque but does hurt top end (these are not good heads breathing wise above 4500 RPM or so). The MPG problem is actually easy to help on that motor in your configuration. The problem is the fuel you are using and timing. That engine loves octane (because of its 9 to 1 plus CR) and responds to it and advanced timing really well. If you fixate on 87 in that beast, MPG is going to be poor at times because timing must be compromised tolerate 87 octane. You want to try setting base line timing from TDC to about 8 BTDC to begin with and use some 93 octane fuel to start (you need to get the octane up of the 87 you have in tank and 93 is a quick way to do it) You will feel a difference instantly when you change timing and fuel and you will find it cheaper to drive on better gas. Example, I have a 89 4x4 burb that I bought new that now has close to 190K miles on it now and in a similar configuration. It averages about 16 in urban driving and a week ago I went on a 321 mile day trip and got 18.1 MPG. It has very crisp response and requires very little throttle pressure to move right out. BTW, you want to run tires at or near max pressure as it reduces rolling resistance and if you live at a high altitude you can advance timing even more. That motor is capable of some impressive MPG when properly feed and timed.  

without a doubt the best advice i have ever gotten. I did this on my 88 burb a few months ago, started running premium, and went 9 BTDC (5000FT) and im getting about 13 mpg in town now w/ 33 in tires and stock 3.73 gears... Cant wait to get it geared  
Where's the time to work on MY stuff?

NoTime:
without a doubt the best advice i have ever gotten. I did this on my 88 burb a few months ago, started running premium, and went 9 BTDC (5000FT) and im getting about 13 mpg in town now w/ 33 in tires and stock 3.73 gears... Cant wait to get it geared

Thanks, at 5000 feet try about 12 BTDC and maybe even 14 BTDC.