Edgar
The fitting of a cross frame to a chassis is a long and quite involved procedure, it vitally important that you measure everything at least 3 times and drill only once. There are others who know far more than I and I hope they will also chip in. It helps to have a companion around, to help hold measuring tapes etc , and even better if you can find one who has already done the job. This a build critical point in the assembly , so if you are in any doubt at all about this procedure (or your ability to measure accurately and drill correctly) I would strongly recommend that you bite the bullet and get Terry Foxen to do it for you.
I am not an expert on this particular job , although I have done it rather too many times (having a racer means you have to expect a few bumps).
Firstly ensure that the chassis is off the ground and perfectly level both fore and aft and side to side, it really helps if this is done on level ground. Check that the new chassis is indeed "straight" . You should keep measuring every angle and length and distance above ground , at the same point on each side of the chassis. Once you are convinced this is OK, fit the cross frame in by hand , a good first position is with the bottom of the cheeks sitting on the lower chassis rail, and the lower tube as far back as they will go on both sides) and keep in position with 4 G clamps (BTW you are going to need several G clamps for this job)
Now measure from the lower cross tube back to the front spring mounting point on both sides and compare the measurements , adjust to suit, until they are the same (or at worst within 1 mm)
Now measure from the top tubes back to the same rear point, compare and adjust to make them the same.
Also take measurements of the bottom tube to the floor (if level, if not apply a correction) on both sides and try to get this the same.
All of this will get you a to more and more accurate positioning of the cross frame, but you must also consider the castor angle. This is the angle formed by the inclination of the king pin, this was specified as 4 Deg originally , but most cars came out at around 2 (in the days before MMC used jigs) which is too low. You should be aiming for minimum 4 Deg and some recommend 6. Whichever you go for, it is again vital that is the same on both sides. This will involve moving the frame front up and down whilst maintaining the fore and aft position, not easy. It may be easier to build up a suspension between the tubes using the stub axle and king pin but without the spring to show you the king pin and you can actually measure the inclination with a protractor, otherwise you can do it with lots of measurements and use geometry to calculate the appropriate angle.
Hopefully by now you should be getting a more accurate position which you are happier with . Now is the time to go back and recheck all the measurements to ensure that everything is still dead on (and adjust if not, it will probably take you several iterations before being happy with the result) If you do have a companion get him/her to measure exactly the same points that you have and compare the results, you really cannot check this too many times. Now put some additional G clamps in place as the last thing you want is for it all to move while drilling. Obviously you are going to drill the holes reusing the old ones in the cross frame, but try to ensure that the drill is truly at 90 deg to the chassis, you really do not want to be increasing or ovalising the current holes. Use new high tensile bolts with new anti vibration washers and new nuts of the exact correct size (obvious I know but you would be surprised how many folk don't) Once it is in, and installed , again go back to ensure that all your measurements are still identical on both sides and hopefully the job is a good 'un. If not, you now have an issue as to why and how this has happened, and you really do not want to be redrilling the same holes, but hopefully this will not be the case.
I hope others will add something to this, apologies if it sounds repetitive, but it is vital to get this right if you want to car to handle well, even a small error can spoil your fun once the car is completed.
Good luck
