I'm pretty sure that you can get a first gen FAT at a point where its YLOD % chance will be the same as that of a Slim. As I bought a 60GB CECHA on eBay yesterday, I've been searching, and found a few tricks to cool down a PS3 (this is mostly for first gens! 65/65 fats shouldn't have troublle, you just have to replace the thermal paste on them).
1/ For CECHA/B/E, swap the PSU for the one found in a CECHG model, APS-231, which is 280W instead of 380, and is compatible with the BC ones (not the CECHC though!). It runs much cooler, and shouln't give any issues as the 1st gen PS3's ned 210-220w at most.
2/ Thermal paste, obviously. Should be done on any PS3 out of warranty.
3/ Check the fan inside your PS3. If it has 15 blades, change it for a 19 blade one (look for an original Sony one, not a 3rd party one!). 17 blade ones should be fine.
4/ Use washer rings with the screws on the bracket that fixes the heatsink to the CPU/GPU. It tightens the cooling block and makes better contact.
Those 4 tips should work very well. Lapping the heatsink so that it becomes flat should help getting better contact too, but it depends on whether the RSX/CELL-BE heatspreaders are flat or not.
I'm going to test all those ideas once the PS3 arrives here. I'll keep you updateed
Also, avoid cutting a hole in the bottom of the PS3 for the fan. It looks like it does good, but I believe it does more bad than good, because the system is designed for the airflow to have an optimized "path" and adding non-optimized holes will mess that up, which might result in some parts of your PS3 not getting cold air and heating more.