Greg,
The T-8 lamps will fit in the t-12 holders, but you have to change the ballast, most of the time. I recently got some ballasts from McMaster-Carr Supply Co. that are rated for either 2 or 4 four foot bulbs and either t-8 or t-12 bulbs. I did not know such an animal existed, but it sure cuts down on required stock, and I always have the right ballast. I quit putting in t-12 bulbs last year if a ballast change is required I always go for the t-8
