The first thing we have to ascertain is the nose weight of the trailer hitch, this should be in the order of 5-10kg. Can be slightly more but certainly no less.
When you place your new engines into their compartment this will send the nose skywards, therefore, before the engines are installed place a 4" x 4" billet of timber between the two rear trailer sections. Place two billets upwright under this. To the side of these billets have two trolly jacks, one either side. Install your new axle but use U bolts which will, therefore, give you flexibility in movement backwards and forwards. Install your engine and the nose should not move. Pump up your trolly jacks, remove the upwright timber, let the trolly jacks down slowly so that the new engine weight is now on the rear axle. Check your nose weight. Simple bathroom scales will do.
Pump up trolly jacks again, move axle until you have the correct nose weight. Tighten up U bolts, weld in place, remove timbers and away you go.