The 55-56 chassis will have the same rear end as the 51 merc except I seem to remember they had different number of axle splines altho were the same width. the cars after the overhead valve Y blocks came out had a different front crossmember and an oil pan with a front sump---probably only being an concern if you want to run a flathead v8

overal, width should be fine, wheel base might vary but all amjor componentry should be somewhat similar

Good luck and keep us advised

Jerry