Make sure you put the bottom engine bolts in correctly, one is 2mm longer and the other is 2mm shorter. Also don't forget the spring washers (otherwise it's like having a too long bolt).
Failure to follow this advice will spoil your day.
Ill admit to this, so as others will learn- I was installing an engine in to one of these in the late 80's and tightened a bolt stright through the lower crancase on the LHS lower front fixing- ill never forget it! One trashed engine!
On the plus side, engines for these were £50 max from the breakers and plenty of them- I simply brought and installed another!
Over the years, Ive had loads of 400/4s...must be something about them! i was in ANC Maidstone this morning and saw a beautiful Imported (USA) yellow one. no expense had been spared on putting it together and it only showed 11k on the clock. It was a customers and Andy of ANC said he belived it genuine.