And yea there is a LT4 with LT5 DOHC head
Mercury Marine made the original LT4 and 5 right?
Well in their marine series of engines they made a engine with the LT4 block and the LT5 heads and upper and lower intake.
They did it to save money and to gain reliabilty.
Its called a LT4 Retro Kit and you can order it through Mercury Marine order center.
It costs $2358.99 and if i can find the receipt ill post a pic of it too.
The kit includes a cam plug and cam bearing oil locators that reroute the oil around the now non existant single cam.
It also incldes the front cover that allows you to mount the puley system for the heads.
And if you were so perplexed you can measure the engine and it is a full 3.81 inches longer than a stock LT4