Re: 5 Bolt 289, NP435 4 Speed... How to?
I feel your pain and this is just my two-cents.
The early 5-bolt block has very few aftermarket options whereas the later, '66 and later 6-block is pretty much the standard for the aftermarket.
This may be hard to swallow, and again only my opinion, but I would recommend finding a 6-bolt block (289 and 302 are interchangeable) and put the current 5-bolt, 289 guts in it. Sell the 5-bolt block to a restorer to make up some $$$ and by switching to the "modern" 6-bolt block, you have just not opened the door for infinite parts. This would be soooooo much easier in the long run.
Racing Cougars (the other Mustang), for four decades...