340 cracked pushrods

Sounds like you are using the old solid pushrods (40 year old parts?) - those solid pushrods arent the greatest, Id do what you plan install some new tubular chrome-moly rods.

I assume you are using the stamped non adjustable rockers ?

If you are using adjustable rockers you need to have the correct lengths, Solid lifter pushrods will be too long. Correct length pushrods are readily available from crane etc.