Crawl under and look at the pan.
Your choices will be limited if you intend to use a 383 or a BB for towing. TH400 or 4L80E if you want success.
As for whether to go 383 or BB, either one will work pretty well. You will have some chance at economy if you go with a 383 and the proper cam. Plus all of your accessories, mounts and brackets will still work.