Grote mfg makes LED lights to fit into old housings. The guys over on Red Power forum recommend these. You can't buy from the mfg, but I see they're available from Amazon. Note that they only draw 1.4A ea!!!

Here's link to Grote specs. Check the dimensions on the spec sheet vs your housings:

http://www.grote.com/product.php?product_number=63821-5

Here's link to Amazon to buy them. Looks like $110/ea:

http://www.amazon.com/FORWARD-LIGHTING-TRILLIANT-TRACTOR-63821-5/dp/B003H0TYIO