I think you will find that the W&B is not an issue, because there is a wide range of acceptable CGs (I'm on my second 337 and have carried unbelievable loads, including 360 lbs of concrete and a full-size bathtub, and never ran afoul or even had to make adjustments, aside from the max load of cargo in the rear. So you ought to get a good idea of useful load if people with P ot T models post their useful loads here (I have a normally aspirated 337G with a take-off gross weight of 4630 lbs and a take-off useful load of 1664 lbs).
I assume you have read the postings here on "What to look for when buying a Skymaster" and also an existing page in Peter's old Skymaster website which deals with this topic at
www.skymaster.org.uk/purchase.asp (it is a depository of information gathered over the years from savvy Skymaster owners to the typical questions asked by prospective buyers).
Ernie