It sounds like a pickup truck 390. Ribs were very common to FE blocks from 69 up thru the mid 70's. 2U is indeed a 390 3.78 stroke crankshaft. And the truck engines used the 410 slugs with it to make a nice low compression engine. The 391 crank is forged steel and has a 1.76" snout dia compared to the std FE which has a 1.38" dia snout. Pretty easy to tell them apart. Also the FE's used a aluminum timing cover except for the very early stamped steel covers. The FT engines had a cast iron timing cover. You can ID a 428 crankshaft by a IU, IUB, IUA stamping on the edge of the counterweight.

Were there heads and intake with this engine?? What's the casting numbers if so?