Garagepunk, there is nothing wrong with the mushroom tappet idea, but many times we look too far outside the box thinking we are gaining a lot, when we are actually no better off that if we used tried and true methods and concepts that have been proven to work and function well. NASCAR used the mushroom tappets in the late 70' and early 80's with good success, but changed to the conventional style flat tappet lifters afterwards with even better results. Cam technology has advanced so far and rapidly, that lobe profiles that weren't able to be had without the use of the mushroom tappets, are now way beyond what could be done with them, using standard tappets. Roller cams have been available for the GMC's since the 50's, so using this type of lobe profile instead of a flat tappet style will even take you to a much higher plateau again.
Flat tappets are more sensitive to lifter bore misalignment than a roller lifter is, so just installing a bushing into the exsisting bores can magnify this problem unless you have precision lifter truing tooling like BHJ offers to do this competently. Plus the additional machining required to install a mushroom tappet into the block, where the bottoms of the lifter bores have to be spotfaced with specialized tooling to allow the lifter foot to recess for clearance presents more challenges. I think you would be money, time and performance ahead by just upgrading to a Chrysler lifter, and having a custom lobe profile ground on your cam to get you where you want to be.
