What on Earth is happening? 'Contaminated' soil??... Are we talking abandoned waste tip with a house on top? or what? How much digging-out is involved? And what boiler is it? (iow, how heavy?)
If the floor joists run side to side under the boiler for a reasonable distance, you could support the weight of most boilers quite easily with temporary props up to (I guess) 1.5 metres away on each side. If the joists run forwards under the boiler and are embedded in the wall behind it, you could probably make do with temporary supports a couple of metres away under 2 or 3 joists. If the eventual objective is to put in a concrete floor slab, I'd have thought the best sequence was to remove the boards, support the boiler, dig out underneath it, cast a small 'pad' in situ underneath it, then do the rest of the room..... (But ensure the DP membrane is properly done.)
However, if pipes from the boiler run under the existing floor, maybe reconsider....