The difference in how deep the flat tappets sit in the roller block doesn't matter, the pushrod length does
A flat tappet cam uses a pushrod that's about 7.8" long. The stock Chevy hydro roller cam pushrod is only about 7.2" long.
The block is a 1-piece rear main seal.
• Valve train geometry
• Valve train points to check
• Cam break in
• Adjusting hydraulic lifters