[Jack21]

I think you will like the extra tq of the bigger motor in the heavier car.

You need to research swapping bigger Windsors into '65/'66 Stangs though. The shocktower braces don't give you a lot of room to do this. One article I read some years back showed installing a complete street rod '74 -'78 Mustang front suspension, brakes, and steering (like many Cobras use as well).

Your rear is going to bounce all over the place unless you install traction bars with the stock leaf springs.

T-10's were nice, but a T5Z close ratio gives you a little more bottom end gearing (2.95 1st), and you'll love 5th for cruising.

Here's another approach. If the butchery you may have to do to your front end and shock towers looks like more than you're willing to tackle, take a look at a 331 or 347 stroker with a Vortech supercharger. You can have late model EFI, and a huffer that thinks it's a 408, and drops back into the same hole the 289 came out of.