Engine transplant

The early 289 engines used a 5 bolt bell housing . I think around mid 65. the bellhousing became a 6 bolt housing. I'm unsure if there was a change in the bolt pattern for your 85 block. I know that the 70's 351W and 302 used the same 6 bolt pattern on their bell housings and should bolt up. If it is not the same pattern, There are companies that make adaptors like Trans-adapt.

as long as you have a 6 bolt bellhousing on your 289(and likely you do since it is a 66), all you need is the 351w engine. you can even use the 289 flexplate on the 351w as it bolts on and uses the same balance factor.

You will need different headers, forf 351w into 66 Stang. If you use manifolds then you will have to "massage" the shock towers so the manifolds clear. True, flex plate will work due to the same balance. You are probably running a 6 bolt bell and that will mate up. You will need a 351w oil pan and oil pump pick-up for front sump, like on a 70 Stang. You might want to swap distributors because the 85 has a different harness or integrate it into an MSD system. If its a roller cam then you will need a billet steel distributor gear. Swap out the water pump because the 85 will be reverse rotation, you should be able to use the 289 pump. Figure out what pulleys you want. serpentine or V-belt. Might want to look into a bigger radiator. I'm going to use a 79 El Camino 3 Row cross flow. I hear that by swapping out cast iron heads and intake for aluminum, the weight saved puts a 351w at about the same as a 289 with the iron parts so the suspension will not notice the engine transplant. Otherwise your front end will weigh more.
I'm doing the same thing but with a T-5 in my 66.