If you mean 90's E-series, they're either 138", 158" (extended), or 176" (probably the cutaway). The two longer wheelbases are E350 only.
The rear end could be either a Dana 60/61 or a Ford 10.25". Not sure which came as the FF in the vans. Pay attention to the tranny - the C6 was still used through '96.
PSD available for '95. (I saw a cargo van with one - it belonged to a coroner /forums/images/graemlins/thumb.gif ) Rear disc showed up on the E250 (w/full-float) & E350 SRW for '99.
81-95 GM vans are either 110", 125", or 146" in full-bodied form. Definitely need the 1-ton for towing. The 3/4-ton uses a 10-bolt /forums/images/graemlins/tongue.gif. Even the 1-ton only uses the 14bSF unless the heavy suspension is optioned. 454 avilable for '89.
96-up is better for towing. True body-on-frame construction. Heavier axles in 3/4-ton. Available 6.5TD. Comes in 135" and 155" wheelbases (full-bodied).