There are threads on here about the part needed to convert LA heads to "through pushrod" oiling, basically hollow pushrods with oil holes and rockers compatible with pushrod oiling; use the search function.
An easier option would be an earlier Magnum block ('92-'94ish) which were still drilled for rocker oiling and would not require any specialized parts; '88-'91/'92 LA roller blocks would also be suitable. Failing that, your machine shop may be willing to drill your late block with the appropriate passages.
