> Will a P90 head bolt onto an L24 block with
> no problems?
Yes it will, however, there are several factors to consider before you do such a thing.
As far as heads that will bolt up, for the most part, the following heads will physically bolt up to a L24, L26 and L28 block,
E-31, E-88, N-42, N-47, P-79, P-90, and P-90A
Now just because it bolts up, doesn't mean you want to use it. The P-79, P-90 and P-90A heads all have a very large combustion chamber design. As a comparison, an N-42 head has about 46cc capacity, but the P-79 chamber style heads run almost 54cc. You can go to my web page, and look on the compression page,(link below), and calculate your approx. compression. To find some of the measurements that are not on my page, ie stroke of a L24, bore of a L24, etc.. check out my link to the Texas Z car club homepage. On their tech section, they have a head combo page, which lists a lot of that stuff.
Bottom line, dropping a P-90 onto your L24 will drop the compression WAY down, not acceptable. Even if you do the head mod described on Bryan Little's Datsun Garage, you still will be too low. It comes down to displacement.
Check them out, and ask if you have more questions.