As far as rare, the 51-54 Chrysler 331 extended bellhousing engines were not produced in large numbers to begin with and relatively speaking not a lot have survived over the last 50+ years. I guess you could say it's reasonably rare

One of the reasons not a lot of these survived is that their not very desirable. This was the first rendendition for the Chrysler HEMI and some of the draw backs included:

The extended bellhousing. This greatly limits the transmission choices. You can use the early 2 spd power flights, the fuid drive transmission, the 3 spd manual units (which were in themselves pretty rare) or if you can find one an early Offenhauser bellhousing adaptor that will allow you to use a 50's style Ford tranny. I have heard of people having the bellhousing extension milled off and the bellhousing flange retapped to use the more modern adaptors for GM/Chrysler trannys, but the cost was more than finding a 55 and latter block.

The heads are the early small valve design and though they were pretty good in 51-54 they were pretty restrictive compared to latter units. Replacement valves are getting VERY hard to find and expensive. These heads can be quickly identified by a lack of water passages at either end of the head.

Because of the lack of water passages in the ends of the head, the engine requires either a 51-54 intake with the cast-in water passages or an aftermarket unit that includes this feature (most do not).

Stock compression on these engines was 7.0-7.5 to 1. While high compression pistons can still be had they are pretty expensive (more so than the 354/392 units).

When all is said and done and the engine is built you have have a heavy, impressive looking engine that is still has only 331 cubic inches.

Because of the reasons listed above these engines do not demand the high prices normally associated with the latter versions.

Don't get me wrong, well done this is can be a nice project that will get a LOT of attention. To the uninformed masses (you know, chevy guys) a HEMIs a HEMI and has all the appeal associated with the name.

Good Luck