The stock cross member on a '51 sits at the aft edge of the 6-cylinder engine. You are going to have trouble convincing me that the small block is short enough for you to bolt the aft end of a 200-4R to the stock crossmember. Do you have pictures?

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.