The suffix code on your 305 is just TU not STU. That can be found on this page as well as the TLS on the 400.
http://www.nastyz28.com/gm-chevy-cod...s-suffix-8.php
The 400 is a far more capable engine than a 305. Most guys won't even bother rebuilding a 305. Even in stock form the 400 will have much more torque than the 305. A mildly built 400 will easily produce 2X the power of the 305.
If the trans in your Suburban is the original trans it will be the 2 speed Powerglide. I personally do not like those transmissions. I much prefer a TH350, TH400 or a 700R4 for an automatic.