In my opinion, an S10 frame (esp. one that is cut and extended will be
unsuitable (or unsafe) for the weight of a 3800.. Too narrow and in need of serious crossmember fabrications.
I would also consider Mustang ll not suitable for this application.

You can really beef up the rear suspension along with a rear end swap without
changing frames and all the attendant fabrications and compromises that entails.
As for the front end ... perhaps consider a well thought out front clip job using a
later model Chevy truck (late 70's/early 80's) that is up to the weight and rolling mass of a 3800. Boxing plates are readily available or easily tabbed up for the stock frame, but unless you plan on 400 or 500 HP twisting up the rear suspension
You'll have a plenty stable platform without all the hassle and uncertainty of frame swapping ... again, just my opinion.

squire