If you are going to rack the pipe then an intermediate board is
almost essential. Some method of retention at the intermediate level
is required, and unless it is just a few stands that you can lash to
the coral,a racking board is the safest way.
Consider the time/cost of tripping singles versus time/costs of
modifying the rig.
Modifications can be expensive and time consuming, and a "Lot of
Tripping" can be done for the cost of rig mods.
Consider tapered string if your upper completion will take 3.5" pipe
which does rack readily.
Consider damage to DHSV (if it is not wireline retrievable)and you
plan to rotate your string during the fishing operation.
BOP, traditionally 7 1/16" BOP is used for this size of pipe.
Consider HP riser from the top of the tree up to the level you want
the 7 1/16" BOP to sit. Insert BOP and on top of that a bell nipple
or xover spool to the main rig bell nipple.
Consider using the same ram configuration as the conventional stack,
(just down sized) for ease of hook up of controls and C/K lines, as
long as it complies with minimum requirement as posted by Augusto.
This helps since no change required to remote panels, or policies or
procedures regards shut in, hang off, well kill line up.