I would be pulling the pan and measuring the original pump and the new one. Sompin' 'aint right, all the small block pumps are interchangeable and big block pumps bolt on with a modified pick-up tube. I'd get a GM or Melling pump and sleep at night with no worries. It's not worth trashing a new motor for a few dollars saved on the inferior part.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy