If drainage is good, then why sand cap? And, I have seen fw improve by topdressing and/or coring out and drilling in sand, usually reserved for greens. It takes longer, of course! Maybe even tilling up and loosening the soil may help drainage penetration. Pending soil tests, you might add sand or additives (although mixing sand and soil sometimes comes out like concrete.) There are also those slit tiles that do seem to help (at least for a while, until thatch covers up the sand and they slow down)
None are perfect solutions, but then again, neither is sand capping. It fits in a whole host of other "innovations" similar to wonder drugs, that turn out to not necessarily ease maintenance, but just cause different maintenance (which might also be a minor annoyance for those 2 holes.)
BTW, whether you cut the sand in, like a USGA green dish, or allow it to feather out, really, really changes the quantity of sand.
Like others have said, if you do it, you need under drains, in herringbone patter, adapted to the topo. You will probably end up adding a few catch basins for surface drainage. You probably won't need 18". If USGA greens drain at 12", why would you need more? 6-12", with most sand capped fw trending towards 6" for cost, unless it's water release curve needs more.
Also realize, the fw will probably block uphill side water from flowing naturally, and you may need some catch basins on the upper side of your fw, which is a good idea anyway. Never want a lot of water to cross a fw, much like road engineers cut swales on both sides of a road, so the pavement only needs to drain what falls on it.
Lastly, its a good time to redesign the fw. You should consider starting it further from the tee (which may necessitate new forward tees) and whether any specific hole needs the full 50 yards you propose, at least for the entire length of the fw.
Not sure you need irrigation as someone suggested. But, at a minimum you need to raise those heads! You will also find those two fw need more irrigation, at least in their formative years, so make sure your current or new irrigation controls allow these fw to be controlled on different stations than other holes.
In short, piece of cake!