Am I right thinking that the difference between a 3.9 block and a 4 litre block are cross bolting of the block itself?
And the difference between a 4 and a 4.6 is the crank, and therefore the stroke length?
So......
Couldnt you, Cross bolt your block, and therefore get the appropriate internal machining done, to allow clearence for a 4.6 crank??
Then, you would have a 4.6 in the original 3.9 block.
(im only speculating btw, i dont think I 'actually' know what im talking about!! )
You could even...
Beg/borrow/steal/BUY(hint hint) a 3.5 off a friend, and get it machined out to 4 litre, with new liners and cross bolted, and then fit a new crank...
However, none of its cheap...