I run into this issue all the time with GURPS. The basic assumptions of GURPS/BRP rules are different than that of the AD&D rules. Unless you are allowing the equivalent of superpowers there is no time that a GURPS/BRP is worth four "normal" characters like a 4th level D&D fighter.
So are two routes you can take. First you can have BRP (or GURPS) simulate D&D dungeon fantasy with the characters able to withstand increase damage. GURPS had dungeon fantasy that starts characters off at 250 pt and in theory BRP can handle it the same way. (Not sure what combination of books will get you there).
The second is to use a regular fantasy character of the system and adapt the module. Most players and referee, including myself, like doing this because these type of characters represent what we like about the system (GURPS, BRP, etc) in the first place.
So how you adapt the module?
Well one way is to look at it realistically. Imagine a world where Hill Giant existed. They would have a society with a range of skills and aptitudes. So the first thing is don't have every hill giant be a skilled fighter. This plays nicely into the skill based system of BRP. So instead of the 12 fearsome Hill Giant warriors of the AD&D original you have two good ones, five that are ok, and the rest are mediocre. Note the original Giant modules were good about including non-combatants like the wives, and children. So they they are well suited for this treatment. So yes the Hill Giants are damn tough but now the highly skilled PCs have a excellent chance of besting them due to superior skill.
Another thing to do is flesh out the locale in the module especially the NPCs. This will provide additional methods of dealing with the situations through roleplay and plans. One of the reason many adopt BRP and GURPS is that character can be good at other things than swords and spells. Create opportunities to do those other thing.
Beyond this you need to be talking about the specific modules. I can try to help if you open another question about which module you are trying to convert. I have adapted several to use with GURPS so I have some experience doing this.