You will need to clarify a bit more on what year your truck is and what t-case you currently have in order for me to tell you whether or not you can reuse the same t-case. I am about 100% sure that you will not be able to use the case with no modifications. I know that the input shaft for these two t- case are different due to the different output shafts used, 10 spline for the SM465 (in some cases 32 spline) and 27 spline for the TH350.
I know that in a NP205 (which I am the most familar with) the two different input shafts use the same bearing in the t-case so it is a simple matter of swapping inputs to make one t-case work with both trannys. I dont know if this applies to other t-cases or not though.
Also if you use the same model t-case you will not need new driveshafts, because the driveline lengths will stay the same.
1973 K20 - 350/465/205