I'm afraid the only lungfish that can be kept in a 55 for life is also the hardest and most expensive to obtain. Protopterus amphibius is the smallest known species of LF. As far as I know, I'm the only one in the US to have one (and it took me 15 years to find it).
Protopterus dolloi can be housed for a couple of years in a 55 as can P annectens. The SA Lepidosiren paradoxa will outgrow a 55 in less than 2 years as will Protopterus aethiopicus. The aussie LF Neoceratodus is a slow grower but will hit the 90 pound range as an adult. It's also as expensive as an amphibius and can cost from $500 wholesale to up to $2400.00 on the retail market.