Peter,
Check out this site and then go to the linked site called Vintage Hotrods. There used to be some sound bytes there, but I can't pull them up.
http://www.btc-bci.com/~billben/flathead.htm

I'll try to answer some of your questions - first, I think a flatty in a 'newer' truck is a great idea, only I'd like to drop one in a Chxxy just to help even the score for all those great old Fords with bowtie power that abound at any car show.....lol

As best I can answer the rest: there are adapters for T5 tranny hookups, also flat-a-matic makes an adapter for C4's. I don't know about the mazda, but you'd need to use a flathead bell housing.
Aftermarket motor mounts - flatheads have the easiest mounts to work with - they are cast into the waterpumps - front corners, so you'd need to box the frame or use corner braces like the F1 pickups, but easy.
HP - the late (49-53's) put out 100HP, Merc's 110. To go to 145 is fairly easy with bolt on mods. Headers, heads, dual carbs or 390 cfm. Heads (those fancy ones from edelbrock or offy run about $400.
Fuel economy? Good guess. Stock flatty would probably be similar, but too many variables to make a guess.
Towing? Again, I'm out of my league. Flatheads with a 3 3/16" bore with a 3 3/4 or 4" stroke means torque at the low end, but not as much at high revs. So I'd guess towing might be ok, but keep in mind flatty's can be temperamental regarding overheating if you're not careful. Check with the guys on the classicar flathead forum for more/better info.
http://www.classicar.com/chatsforums/forums/messages/default.asp?nSubGroupID=2119&nAdID=94778&strSGName =Flatheads+Forum&nGroupID=1025
Great question! Flatheads are basically expensive to put together, so the whole idea of making a low buck unit might not be compatible with your goal. Hope some of this helps. Let me know if you go ahead with this....
Tim