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Im going by yo7a66 post 8-04-19. He had the 1-2 shift thump at low throttle too. Carburge and dave rs23 said what you are feeling is vb not releasing the front band before applying psi to the front clutches that causes that problem.
You yourself (John Kunkel) said lots folks consider the orifice restrictor a band aid, but if it works so what and its the cheapest and easiest way to mitigate overlap. Start of small at .080.
I think just getting rid of the shuttle valve blocker will help a lot. The70 factory manual says shuttle valve !'st function is fast release of kickdown band and smooth front clutch engagement on light foot upshift from 2 to 3.
Way back in 3-21-09 u (J K) said with r band apply max flow often causes severe overlap on manual 1-2 shift since rear band cant release as quickly as kd band applys . U said leave d ball in place and adjust the size of the servo feed orifice for good shift without overlap. ( Whats the servo feed orifice?). You said without the accumulator it can shift so hard it can shear the aluminum front pinion.carrier splines. Then you said jack r end up and shift 1-2 at 1500 and tires will stop for a second. I think mine did. Hard to remember 14 years ago. Plus i have a sinus infection? I think i might just go .140 on the trans case that yo7a66 used the factory manual calls it front clutch apply. He used >093.
You're taking what I said out of context i.e. you're asking about an "orifice restrictor" to cure both 1-2 and 2-3 overlap while the restrictor deals only with the 2-3 shift. Depending on the year (which you didn't state) the 1-2 overlap is usually addressed with a heavier return spring in the rear servo. In the years with a restrictor orifice in the rear servo separator plate, increasing the size of the hole will mitigate the problem but you state you have an aftermarket plate so that should already be the larger size.
Keep in mind that the front clutch orifice in the separator plate varies in size over the years so, in some years, the orifice in the plate
is the restrictor but, again, if you have the aftermarket plate, the plate will have the larger orifice and a restrictor orifice in the case (supplied with a TF-2 kit) will help the 2-3 overlap. The orifice in the kit is a cup soft plug with a drilled hole but for home brewing it's common to drill the case and tap it for a 1/8" NPT plug...drill it with the orifice of your choosing .080" would be a good starting size.
