The strain doesn't matter in my experience. Only difference is alkaloid levels. For me taking one strain of kratom is the same as any other, and yes I do trust my source. So with that info, if you do feel different effects (energy as opposed to pain relief) then I suggest a white or green strain to help with your energy levels during wd.
I would wait (it sucks) until you are in full wd to start your kratom. 24hrs or so from my experience. Having a heroin tolerance, id start with 3-5 grams. Personally I bought a little capsule machine and take it as capsules because kratom tastes like ass, and I cant do the toss and wash method (toss your GRAMs of powder in your mouth and wash it down with whatever). For purposes of wd symptom easing it should help to some noticeable effect. It will not take away all wd symptoms, but will ease them more than pretty much anything, aside from opiates lol (it works as an opioid type chemical, without being structurally similar to opiates (any more knowledgeable Blers chime in!)
So as an internet expert on drug abuse, health, and psychology and women's tits my prescription is as follows:
Wait 24 hrs from your last heroin dose to take your kratom. Actually plan this out. The more prepared you are the better your chances are of success...only gotta do it right once. Have comfort meds, easy to cook healthy food and drink and whatever else you think might help...loperamide, benzos, cannabis etc. Once your in full wd take a 3-5 gram dose of kratom (I think according to erowid this is a high dose, but Im accounting for OPs previous opiate tolerance, so a little consideration cant hurt). Considering you probably need to eat and hydrate, id do this about 15 mins after dosing if you can, as kratom often depresses my appetite. You should start to notice a bit of relief by an hour in. If no relief is noted, wait an hour or two and take another gram or two. Its not a catch all, but a lot of people do report wd relief to the point of baseline or better, or even getting high. Then some report no effect what so ever. So go into it not looking to substitute getting high on heroin (that's why I suggest against the red veins as theyre most "opiate like" if you buy into that school of thought.) The goal (at least in my head) is to get off opiates, so youll only need kratom for a few days.
Long term kratom use will cause withdrawal similar to opiates after cessation of use, so use caution....it is very easy to substitute addictions. Let us know how it goes op!