Most things like this are money so all reasons have to do with that. The amount you would spend fitting the lifters will far outweigh any benefit. They would potentially be a detriment since the larger lifter face of the stock Y lifter allows for a more aggressive profile - one reason mushroom lifters are sometimes banned in competitions. A retrofitted roller would be great if it could be figured out, but still with an impractical cost.for most people.
There is not enough material in that area of the block to do that easily.
Hydraulic lifters would be a bigger undertaking because you would have to figure out how to get oil to the lifters so they will function correctly, as well as figuring out how to fit them in the first place.
Lawrenceville, GA