The 79 RM forks are 38mm and the steering stops are different. I am not so sure they will work with the frame stops without some additional mods. Stems are longer too. All the ones i suggested are bolt on's, but if you want 38mm then i guess you gotta go with 79/80 stuff then. The 38mm forks are longer than the 78 36mm forks too. I would carefully consider fork length thats for sure. Its not just a matter of chucking the extra length up out the top triple clamp which will reduce the gap you have here when full bottom.
Here are RM370 forkss you can see there is no way these can be slid up anymore. They only have a 22mm gap as standard here between slider and base of triple. This is what you need to check out, if intending to slide the forks out the clamps to reduce length.
Consider the how long the stock forks are from axle hole to top of the chrome and then look at what ones you want to use. The issue is, if you go a lot longer in front and at the rear it does raise the center of gravity and ride height a lot. The bikes that these long forks may come from have all this factored in and the engine cradles sit down lower. As soon as you get into this project you will quickly learn how one thing effects another and you will need to do a lot of measuring, experimentation, trial fitting of parts etc. It can become very time consuming if you really want it to work out well. My advice is don't go to extremes with the mods and take your time with it. I would be looking for a set of forks in the 9 inch travel range roughly around 840-870mm long. The previous mentioned issues of sprocket/pivot location and distance between top shock mounts and swingarm pivot are going to limit what you can do with the front anyway. Sprocket sizes also affect things. Larger ones will give you a bit more breathing space at the buffer area but do they give you the ratio you are needing...... decisions,decisions. I hope you like spending hours in the shed
