My friend Chris Ellis fitted an XT 500 engine to a '77 Husky frame and the conversion is pretty straight forward. He brought the steering head in 1.5 degrees and fitted '79 or '80 KX125 38mm triple clamps and forks (
the first Uni-Trak 125, whatever year that was ) and used an RM250C2 aluminium swingarm. He also used the KX wheels, front and back, the KX seat and the obligatory Yamaha YZ 125X tank. The bike was almost finished until he bought a GMC HL kit so the Husky frame is now being put aside.
a '78 Husky/XT at the TT Muster.Huskys are 'famous' for being slow turners but unbelievably good handling machines. Huskys definitely need some rider adjustment when changing from a Japanese bike or worse, from a Maico as I did. If you're the type of rider who sits in the one spot and doesn't move around a lot, you'll pretty quickly find yourself moving as far forward as you can in flat corners and berms without even thinking about it.
Back in the early 90s I wrote a lengthy 4 stroke class 'shootout' for ADB that never went to press in its full form due to some weird photo quality issues. During the shootout / photo session we compared the handling of a '73 400 Husky and a similar bike fitted with a 350 XL Honda engine. To our surprise the Honda powered bike turned much better than the stocker. The bikes had virtually identical suspension setups (
the same bloke owned both bikes) yet they felt totally different, most notably in corners. A number of different riders including both GE and I tried both bikes and we all came to the same conclusion, that the 4 stroke bike was noticably nicer to ride. GE had a rabid dislike of 4 strokes at the time and it was funny seeing him reluctantly agreeing with the general concensus. For want of any other reasons we came to the conclusion that the extra weight of the Honda engine over the front end had the same effect as 'body English', moving up over the tank during cornering. We noticed the same situation, albeit to a slightly lesser degree, on Anthony Suttors XL250 powered Mk4 'Boat tail' Pursang.
If the extra weight of the extremely lumpy XL500 Yamaha engine has the same effect on the late Husky frame I doubt you'd need to do any steering head modifications. Triple clamp offset, fork length and axle location also have a bearing on the bikes handling/turning ability but to my thinking, I'd leave the rake as is unless you find the bike totally alien to your riding style.