I use the B50 shortblock in my engine (same shortblock as the crate engine) It takes a beating well... The Mustangs Plus cars have the 285hp version in there race car as over 6 years of nothing but racing.
I've got the XB3 long block with a 600cfm Edelbrock and Edelbrock RPM manifold. It took a little extra tuning to get it set up, but it is a GREAT engine. I only have about 400 miles on the car / engine at this point, but no problems yet.
You'll need 20w50 or 15w50 oil and 93 or 94 gas, BTW, to run the car regularly on the street. My dyno figures are lower than I had expected (about 250 hp and 270lb-ft,) but I didn't "follow the directions" and install it with an EFI system.
Its the best and most bang for your buck unless 1)you've hit the lottery and money doesn't matter. 2) You own an automotive machine shop(or daddy does) and you have unlimited engineering resources. 3)you actually like to spend three times as much money and time rebuilding a boneyard block with mismatched parts that will probably not run as well as you had planned.
Have carbed 270hp Motorsport 302 in my '65. Runs strong with T5 and 3:50 gears. Fairly good fuel economy too. I can recommend it. Idle lopes a bit too much for my taste but I've learned to live with it.