Build up ideas. 69 fastback, 390FE w/ T-5

Plenty of cheap 351w blocks out there. Build yourself a nice 408.

I would bail to the 351. The 390 carries a significant weight penalty and really don't crank out the power to offest it. I had one- on my engine stand it was so heavy I was worried about the stand and used a block of wood under the pan to assist.
They are expensive to build and leave you with little header room. If you do stay with an FE, stroke it out to maximize your gain. There are some interesting 428 and 427 crank and block combinations that get you around 455CI IIRC.
For what it's worth, I bailed and built a stroked out Windsor to 515 ft lbs of torque.

Not only that, but you can use the motor mounts you have, and the tranny conversion will be easier.

Blocks? How many good blocks do you want? Only way I'd consider a 351W would be a roller block. If you car is an original 390 car, stick with the 390 or find a 427-428. Converting it to a small block will cost you.

I would stick with the 390 for obvious reasons (I have one). Good used 390 blocks go for $150 to $200 where I’m at. You can build a short block for $1400 to $1500 using good parts. After that, add Ebrock RPM heads intake and water pump another $1500 and the 390 will be about the same weight as a 351 with iron heads. The above combo should get you 350 to 400hp depending on the cam. You can bump that up if you get a port job on the heads. Another option is buy an already assembled 390. Jasper sells a stock replacement for $2800 with a 75,000 warranty. There is a shop that sells on Ebay that I was considering. $3400 delivered if I remember right. 425hp 390 ready to go.

Something to consider, the 69 factory “S” code Mustang Mach 1 is the rarest Mach 1 build that year. Less of them were produced than 428 equipped Mach 1’s

Tim