Wilcap is a long time supplier of adaptors. I don't know about your application. Someone here will know if that trans will live behind that motor in a full size car, or the rear end. I've broken both with flatheads, in my younger, more foolish days.
You can get a ton of information here:http://www.jalopyjournal.com/forum/group.php?groupid=332 regarding your swap, we have a HUGE "what fits" file available there.You will need this too:http://www.butchscoolstuff.com/store/cart.php?m=product_detail&p=16 Note: there are two different kits to the right of the small picture click on it and a chart will show how to measure for the correct one.The rear end can probably handle a mild 351W with an Auto trans your 3 speed stick won't,also your stock rear is a 3:90 gear your 351 will be turning some revs and sucking gas at highway speeds.Best bet is running a 1988-93 non-computer AOD behind the 351W it can take the HP and with the overdrive you can cruise EZ.