I'm going to disagree with the approach SwingDaddy suggested. While the '54/'55 1st cross member with a later bell housing is a good approach when using the original motor I think in a case like yours there is a better way.
I didn't say it couldn't or shouldn't be done. I have very little experience setting up hydraulic clutches, so I said I wouldn't go this route, and when I said I, I actually meant me personally. It actually seems like a pretty straight forward swap, as the 250 should bolt right up to that transmission. Again this is not my area of expertise, and I haven't done this swap, but I would assume the bellhousing crossmember will need to be removed, and another crossmember fabricated, as well as sidemounts for the engine. I realize this is done everyday when swapping a v8 with an automatic transmission. The frame may require extra strengthening to cope with the loss of that crossmember.
The route I mentioned only has the advantage of keeping a crossmember in the original location, and possibly using the stock clutch linkages. The v8 bellhousing would bolt right up to that crossmember. Again... I haven't done that swap either, so it is still not my area of expertise.
You have the transmission I say go for it!
