My thought is that the short pump is where you are having issues. The short pump was used in early applications (1960's) and in the Corvette up to maybe 1980. The long pump is by far the most common on all the small blocks with brackets and pulleys being designed for mostly the long pump. I would stay with all long pump equipment, the difference between them is only roughly 3/8" in depth.

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