I've often thought a Final Fantasy 1 conversion might be fun myself. Many of the monsters and spells are D&D homages or straight ripoffs and have direct analogues. You'll also have to do something about the HP curve as Chaos has 2000 HP and you can't give a monster more than 255. The class change would be one of the hardest things to carry off--Bahamut could just raise everyone a level.
For Bard's Tale, you're going to be stuck because FRUA has 3 classes of spells at the most. My advice would be to turn the Ranger into a Bard with the Druid spells hacked into the buffs the Bard had. For the Monk, use the unimplemented Knight class and transfer the Ranger Damage Bonus, make sure it looks at Knight level, then give every monster the Ranger Damage Bonus dis-ability. Don't even bother with the 4 spellcasting classes--just rename the Cleric and Mage to Magician and Conjuror. You also only have one summoning spell, so I'd make it first level and let the monsters increase in power with dungeon level. There are only three spells that give abilities that can be checked for--Detect Magic, Detect Invisible, and Find Traps--but you could easily hack, say, 'Find Traps' to 'Summon Humanoid', and have an event called in every zone that checks if 'Summon Humanoid' is cast and then adds a level-appropriate humanoid (kobold at the beginning, storm giant at the end) through an Add NPC event.
Also, Skara Brae is about 30x30 so you will need multiple dungeons.
(Also, at the risk of plugging my own awful work, Vanilla Dungeon does have an optional fight with every default monster in FRUA...