Umm... I am not qujte sure you want the T-34 as a second plane. Warbirds are somewhat different and require a tad bit more experience when flying them. They are much more agressive and tend to stall easily. You may want to try something like a Tiger II, Four Star 40, or a Tiger 60 as a second plane. You can get the performance out of them plus they are much more stable than a warbird is.

But to answer your original question, if you want to slow the plane down, change the prop pitch a bit. 10-6 to an 11-4 or something on that order. But, the pitch change will slow it down a bit.