1978 Huskvarna 250 frame HL500 Project
I get allot of emails from folks trying to decide which frame to use on their HL project. I found a 1978 Husky 250 frame and decided to see how easy a XT500 motor would fit. From these pictures it is the way to go. Engine drops right in with no frame mods. The diagonals that attach to the main tube under the tank will have to be removed and relocated to clear the carb but is a minor change. I have seen Husky125 frames without those bars.  I also tried to fit the engine in a 78 RM400 frame and the whole front downtube will need to be cut and moved forward two inches and lowered two inches, allot of work! So the ticket is a late 70s Husky frame, similar to the one Bengt Aberg used in hte original HL500. Next I will see how the RM400 aluminum swingarm fits. The problem there is the Husky is a right side chain engine so swing arm is shifted right to clear chain from shock. The RM is a left chain so shock mounts will probably not match.


Husky Frame Info
by Jim Smith
The ML frames are the least invasive to build an HL  upon. ML frames ( Ed Gocek build) were produced on first the 250/360 GP models, in 1976 covered all off road models thru 1978. I recently purchased a near scrap 1978 250WR frame from for the purpose of harvesting the timken bearing cups for my 1978 390 AMX frame. After adapting them and pressing them in place of the ball and cone races, I found I was able to use any Husqvarna front end and adapted a set IT490 forks with 1988 KX250 triple clamps with a bearing from a conversion kit. Kinda cool looking but seem to high.. The frames like your's were built 1979-1982 with MM,MN,and WN, CN, and XN. The MM was for 1979 only and MN was 1980, and CN,WN, and XN were 1981-1982. 1983 saw the -O frame introduced that has nice geometry but requiring a cradle stretch like you did with your Maico HL500
 
From Husky Club:

Frame numbers - 
Where are they? What do they look like?

All Husqvarna frame numbers are stamped into the frame. 
For all model years 1963 thru 1999 they are stamped into the steering head area.

for 1963-1970 they are stamped into the left side steering head area and begin with year of manufacture . Example would be  66217 or 671235 or65027. Engine and frame number would match for 66 and 67 if the bike is original. MF, MG or MH (68, 69, or 70) frame numbers will not match engine number or even be close. 
  
 

For model years 71 thru 86 they are stamped into the right side of steering head. they would be like MK01235 for 1973,or MM1235 for 1979 or MN1993 for 1980 or ML12778 for 1976 or ML23995 for 78 or CP08045 for 86 or XP 03456 for 85..... and so on . refer to my model number page. look at my book page for owners manuals. Engine numbers did not match frame and are not same type numbering system. 
In 1981 Husqvarna incorporated a letter into the frame number that identified the "type" frame which helped identify the bike quicker. A 81 frame number would be like CN 1023 or XN 1024 or WN 1025. the "C" meant close ratio mx type, "W" meant wide ratio Enduro type with lights, and"X" meant cross country no light but with lightning coils generally wide ratio engine in a mx chassis type (kind of).

I can't find my frame number or it doesn't look correct

Causes - 
1. You just lack experience, be patient , look the steering head over very carefully. 
2. It has been removed (ground off) because of ownership problems...duuuuhhhh this means stolen . 
3. Frame replaced with new part and not restamped by dealer. (this 1 out 10,000, don't let somebody bullshit this one) 
4. It is painted over.Or powder coated over 
5. You don't know what the "steering head" area means. This is the frame area where the forks pivot. just turn the forks to the limit right or left and look at this area of the frame. Not the upper or lower triple clamps. 
6. Sometimes the owners manual, in some cases, explains this stamped number as well 
7. It has been ground off and restamped to make it more valuable. Be aware of this! People love to make all kinds of claims about motorcycles - first, last, Steve Mcqueen owned it, Hallman rode it, factory works bike. 
8. It has been ground off and restamped into the other side. 
9. something i haven't thought of