Curiously enough, many of the cam lobe shapes in major cam mfg. "library" designed for use with mushroom tappets were originally for a very obsolete engine: the Ford Model "T" (a sidevalve L4) with very bad breathing but very compact and rigid valve gear. The Model "T"'s .970" tappet foot was re-cycled for use in other engines.
This means that an existing lobe profile design for the .970" mushroom is safe to use in the stovebolt and GMC engines with their .990" tappets (a narrower tappet would dig in to the side of the lobe). A wider foot will not change the lift or duration, but will allow far more violent acceleration in lobe design - the rising lobe contacts the leading edge of the tappet foot significantly ahead of a smaller tappet, and has a longer path before passing under the trailing edge.
Compared to the usual SBC/Gen-3 .842" tappet (frequently used as the model for current stovebolt cams - ask your grinder what it's based on), the .970" tappet allows about 15% greater maximum velocity.
The roller wheel on a tappet has 2 functions:
1. anti-friction, no break-in, much less chance of scuffing, etc.
2. completely changes when & where the tappet moves vertically w/r/t the lobe's rotational position; a flat tappet (solid, hydraulic or mushroom) always follows the highest point of the cam lobe, while a roller tappet (solid or hydraulic) always follows the closest tangential point on the lobe (which will differ slightly with roller wheel diameters).
The downside is very expensive heavy tappets and a billet cam.
