On the T 34s rebuilt with the Panzer III cupolas, looks like the original Russian periscope was retained. Looks like it would block the forward view port of the cupola. Am I missing something?

In short: NO.

There are lots of pics of "Beute T-34", and of "T-34 Pz.Kpfw 747(r)" - The latter actually including T-34 assembled by the Germans at captured plants. Many pics show all models of T-34-76 (from mod. 1940 thru mod. 1943 as made at several Soviet plants, plus some T-34/85) in German service. These tanks include all versions of the original Russian top hatches, including the early 1-piece hatch, the round "Mickey Mouse" paired hatches, and Russian-built mod. 1942 and 1943 tanks with the Soviet command cupola (ash-can cupola). Many captured T-34 were fitted with salvaged Pz. III cupola (the version(s) you are asking about).

And YES, both the original Russian "ash-can" cupola and the German Pz. III cupola appeared in conjunction with the pair of original tall Russian roof-mounted periscopes - and both cupolas actually found themselves behind and partially - blocked by these periscopes! When the ash-can or Pz. III cupola was fitted onto a turret with those periscopes - the periscopes - NECESSARY for the gunner to lay his gun - remained in place. (I guess it was a sort of "commander-be-damned" moment!) .

Note that the tall periscopes appeared on some but not all T-34-76 versions / mods built between 1940 and 1943. When present, they were kept on the captured tanks. So you can see Pz. III cupola "Beute T-34" with and without these tall periscopes.

So, kits of Beute T-34 with Pz. III cupola setting behind tall roof-mount periscopes are realistic, odd as it may seem!

Cheers! Bob

PS: Google "T-34 tank in German service" and view the many photos!