The two approaches stemming from "FBI" territory to accomodate 4.125"* sleeves are caging the sleeves via the Haibeck fixture, or the AES Racing's closed deck sleeve system; both requiring considerable machining to the block.
*AES Racing claimes 4.165" sleeves possible yielding a 450 cid LT5.
Haibeck's open-deck, caged sleeve approach:
AES Racing's closed deck approach:
To my knowledge, there hasn't been enough LT5 samples of either design built to qualify yet as a valid sample size, far as reliability testing goes, except both concepts have been have been significantly vetted in other applications. And, the existing LT5 samples of each design have proven very reliable to date; examples of each making well over 680 NA chp!
P.