Hi, and welcome to the forum.
While it's assumed Blackmore just used a "Strat into a Marshall", he's had his amps heavily modified for much more gain and used hot single coil pickup, particularly in the Rainbow period and to the present. He's said he wanted to get the "Vox AC30" sound from his Marshalls and is rumoured to have actually built an AC30 into one of his 4x12's (I've read where he's said he only used a Marshall stack since it was 'the look' and had the power for big stages, but much preferred the tone of an AC30).
It's also been rumoured he used the AC30 as a preamp out to the Marshall heads which may have some truth to it; I've seen pics of an AC30 behind his Marshalls. Can't say for sure they are connected but the AC30 has wires/connections and seems to be in use on his rig. He also used a reel-to-reel tape recorder before the Marshall head for a 'large clean boost' to hit them harder.
As well, his Marshall Majors were apparently modified with another tube preamp stage for more dirt in the preamp.
So, strange as it may sound, you might want to try getting his fat Rainbow Strat tone using a hot single coil, and AC30 with a treble boost, into a Marshall 4x12 cab/IR. Or maybe using a tube drive into the Marshall 100W Plexi Jumpered and using additional EQ, pre-boost, etc. to get a fatter, higher gained tone ala Rainbow.
