Your problems are:
1. The position of the front axle and the fork offset that the R65 has built into your yokes.
The R65 fork leg axle is mounted down the centre ine of the fork.
The R80 (of a similar age) the axle is mounted on the front of the fork.
The fork yokes (triple trees) on an R65 compensate for this by having more fork offset - I.e. the fork legs are further forward in relation to the stem. both bikes have a similar trail.
If you fit fork legs from an R80 into yokes of an R65 you will decrease the trail by a significant amount but also increase the wheelbase. It will affect the handling, but by what margine would be difficult to say, you will knock off 1/2 to 3/4" of trail off your bike, taking it down to aprox 3".
I have a spare R65 yoke set-up that I have been threatening to put them on my R100 - not only will I change the trail, but I will have to change the brake disc positioning as the R65 yokes are wider as well.
2. The legs themselves have different lengths of travel, although if you measure an R80 and R65 togehter, side by side, when the bikes are at rest, with the same weight on wheels, you will find the lengths very similar, it just means that the R80 forks will extend and compress further. So mixing parts probably wont work.
I find that $400 is expensive. I have recently had mine refurbished on my R100 for less that half that in £s (£160 for the pair), and we no longer have a 2:1 relationship between $s and £s.
I would stick with the R65 forks as they are better anyway, and so is the yoke arangment. They do not call R80s and R100s Rubber Cows for nothing!
Cheers
Steve
