With "fast" do you mean tip action or modern rod which often needs to be overlined because it is not rated like old rods were?
If tip action I don't see much difference because with them I use wider casting stroke and accelerate smoothly to avoid tailing loops. You can change rod action slower shortening blank from the tip. Cutting about four inches makes tip action rod medium action but this also require about one weight class heavier line to be cast to to cause more bend.
You can double hand test cast single hand rods before deciding to build proper handles. Few inches bottom handle length you miss should not mean much. Put reel or line spool to your pocket and you can comfortable hold bottom hand from the reel seat. For top hand grip I form a circle with thumb and two first fingers with Spey rods anyway so guiding only thin blank is not an issue for me.
I have DH and SH fished blend style between flyfishing and Tenkara where I tie 30ft of shooting line to the hook keeper so I can shoot line and my fishing range is better than Tenkara and also can play fish classic way hand on line. So although I have built Trout conversion rod I don't bother anymore if I want to DH cast SH rods. For big fish I use 7wt which has a fighting butt and I can keep reel attached and double hand cast.
The thing is that I like to DH cast heavier line which also has shorter head because DH casting is Spey. For single hand casting same line OH and Spey casts fine when I can use hauling to adjust casting
If "balance" comes to someones mind for me it is just one of the myths in fly fishing :smokin:
Often "fast action" is a marketing term (BS) which does not say rod has tip action but tries to give an impression of a rod which "loads" easily and then "recovers" fast so it slings line unbelievable far. Such technology does not exists in current rods!
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