Unfortunately, pushrod length on a y-block is not simply a matter of the engine displacement size.
What Greg was referring to is that different year FoMoCo y-block cylinder heads in combination with different ratio rockerarms (1.54 or 1.43 to 1) result in two (?) different common pushrod lengths for y-block engines.
Google 'John Mummert' and review the pushrod offerings on his web site for more information on this.
The tubular pushrods are desirable for reliability purposes only. They will not increase horsepower.
Hope this helps!

Regards,
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York