This paper specifies how to combine D&D 3rd Edition (©2000) with Boot Hill 2nd Edition (©1979).
Movement and Scale
Fortunately, movement rates for the two games are completely identical. A human can walk 6" (30 ft.) per turn, run/dodge (double move) at 12" (60 ft.), and run 24" (240 ft.) per turn. Western horses are all the equivalent of light riding horses (medium encumbrance with a normal rider), and so have a movement rate of 8" (40 ft.) walking, 16" (80 ft.) trotting (double move), and 32" (160 ft.) galloping (running). Directional changes have some differing rules (Boot Hill only restricts horses; either rule-set can be used for convenience).
There is some minor difference in distance and time scale. D&D presumes a 1"=5 ft. tabletop scale, and a 6 second turn. Boot Hill is considered to have a 1"=6 ft. tabletop scale (note that the prepackaged maps are at a smaller ½"=6 ft. scale), and a 10 second turn length. They are close enough to be considered the same.
Characters and Combat
In the original Boot Hill rules, characters are supposed to be converted to 2nd level human Fighters for D&D purposes (under current rules, having 4 Feats each); it may make as much sense to convert them to 2nd level Warriors (with 2 Feats each), or other combinations of NPC classes (unless the PCs specifically have a background of Army, Deputy U.S. Marshal, or Texas Ranger training or service). Choices such as Alertness, Exotic Weapon Proficiency (firearms), Improved Initiative, Point Blank Shot, Rapid Shot, Quick Draw, and Weapon Focus may make the most sense for standard Boot Hill characters.
Rate-of-fire differs between the two systems. Boot Hill allows a character with the right weapon to fire 3 shots per turn, and bows are fired only every other turn (counting reload times); while D&D allows bow fire every round, and 2nd level characters would need the Rapid Shot feat just to fire 2 shots per turn (and require a non-moving full attack action, as well). To solve this discrepancy, I recommend giving any Modern or Futuristic weapons (see DMG p. 163-164) the special ability to take one shot in any round as a free action, basically just like the "Speed" weapon ability (thereby giving a possible 3 shots per turn to gunfighters, with a single shot during a double move or even a run action [at -4 to hit if running]). Allow reloading via a move-equivalent action at the rates given for the Boot Hill weapons (since the rules note that this can be accomplished at the same time as walking movement).
Although Boot Hill indicates that D&D armor is somewhat effective against Boot Hill weapons (on p. 31), this modifier only works out to -5% (that is, 1 in 20) per 10 points of armor bonus. Thus, it is just as easily ignored for firearms, giving all the Renaissance, Modern, and Futuristic weapons the equivalent of the "Brilliant Energy" weapon special ability. Indeed, in action these items appear very much like "fire sticks" to the uninitiated.
Use the rules on DMG 162-164 for damage dealt by modern forearms. The damage has been increased by one die type over the guidelines given in Boot Hill rules p. 31, and all such weapons have a critical bonus of x3.