Dont waste your time with extensions....
Sell your rails to someone who wants to extend their sled to a 159 for about $200 and buy a set of Ice Age Rails for $300 delivered... you'll be way ahead in the strength and weight department and have a much cleaner look too at about the same price.
Just a few years back... Rail extensions were the only way to go for longtracking.... but now, OEM or better quality rails ARE available at a VERY affordable price.
The 174 only comes in a 2.5" tall lug.... you'll need to D&R the chaincase for that. (Carls may have some Holz D&R's available still for that chassis OR... Maybe you can call Holz directly and see if they have any unsold ones (no longer on their site)... worth a try.
Email Mike Johnson at Ice Age...
mike@wiktel.com
He'll set you up... tell him Mountainhorse sent you.