D&D damage scale is roughly one-third that of Tunnels & Trolls. Use only the Small/Medium damage ratings in games like AD&D/OSRIC when doing this multiplication or division. The v. Large Creatures modification throws this off kilter, so don't use them. Bonuses or Penalties due to Str or Fighter Exceptional Str are also modified by a factor of three. Magical bonuses also are affected in this manner.
When converting from T&T to D&D, there will be weapons that have 'Adds', a flat bonus beyond dice. First divide these by 3.5 to determine additional dice; only whole numbers count. Any remainders of this step should be divided by 6, and if 0.5 or greater round them up as a single +1 to be dealt with after division by three. A lot of work? So are tracking XP.
What this means is that a 6+2 Grand Shamsheer (No-Datchi) sword in T&T 5e will be a 2d6, or if one prefers, 1d12 sword in D&D. Two or more dice not only have a higher minimum, but an average value, which I like, but D&D sensibilities may run counter; YMMV.
Ignore Too Heavy Weapons, and in general, the STR and DEX requirements if dealing with D&Ders going into T&T.
ARMOUR CLASS & HITS-
In T&T, the weapons dice are rolled, personal modifiers added, and the total compared to the opponent's total. The loser of this comparison takes the difference. If the target is armoured, the hits come off this first, and then any remainder to Con. When Con hits zero, the target is dead or effectively so.
In D&D, the armour class comes first, determining whether the strike penetrates and reaches the target, at which point the damage is rolled, and is taken to Hit Points. In some versions of D&D, Con is figured in the Death Measure, if you will, to determine when the target is fully dead, but at anything below 1 Hit Point, the target is out of the fight. What is going on here? The same thing.
Subtract T&T Armour Hits (before doubling) from 14 to determine the Descending AC in D&D. Values higher than 10 count as 10 AC. To use T&T with Ascending AC (DCC, 3.x, Pathfinder, etc.), add the T&T value to 10. Going from DAC to T&T, simply consult the armour type and adjust accordingly.
Since in some versions of D&D, and in T&T, Con is the fully dead measure, just use that and be done with it. If someone takes less than (HP + Con), then they are still up for the fight; more than this, they are out, and likely, dead.
Monsters in T&T recover 1 Hit per 2 Rounds, or 5 per Turn. This is important.
Divide MR by 8 to determine the creature's Hit Dice. Once HD has been determined, use the D&D HD Attack Matrix to conduct monster combat, and to track Hit Points. I'd recommend giving max HP, but I'm mean. Going from D&D to T&T, multiply the HD of the creature by 9, yes, nine, to determine its MR. Carry on as usual.
Special Attacks / Abilities. T&T rocks in that great latitude is afforded the GM to determine how to flesh out their critters. If the D&D creature has a breath attack that does 3d4 (12 HP), it gets a 6d6 Fiery Breath in T&T a number of times per day equal to the D&D source. Spell-like Abilities in D&D are just things T&T critters can do because they are monster/things. Going from a T&T source that mentions Trigger Numbers (Gaze on three 6s, for example), means that there is a percentage chance that each Round opf combat, someone will be subject to a (Gaze attack in the above case). If D&Ding this, just use the closest equivalent rather than determining the percentage. Maybe only on Critical 'Hits', or if the Hit is an Odd Number, or roll 1d6 and on a 6 or a 5+ or a 4+, etc. the ability or attack activates.
I'll get to Magic in the next post.