Bill - Preliminary research indicates the block casting# 3994257 is probably a 1967-72 Chevy 250.
In your 3rd picture, the date casting code is partially obscured by the dip stick tube. There should be a first character "letter" (currently hidden behind the dipstick tube) followed by a space and one or two numbers, then a space and another single digit number. I can see a "7", space and a "2" to the right of the tube and those should indicate the day of the month and year cast. The first letter indicates the month of the year cast (ie: A=January, B=February, etc). From what we can see the block was cast on the 7th day of the yet unknown month, in 1972.
For the best/most accurate information on what the engine is and what it was installed in originally, we need to see the stamped serial number which can be found on a machined surface pad just to the rear of where the distributor goes into the block. If you can clean that up and add a picture, we can help decode the stamping.
