I'll take a shot at this as well. My 47 has no original frame, but the standard motor and trans mounting for the 30s through the late 50 Dodge trucks were to have a single front mount under the front crank pulley, attached to a crossmember and a mount on each side of the bellhousing. The trans hung out in the breeze.

The flathead 4 or flathead 6 were the standard motors available, and there was a long and a short version of the 6. The firewalls were either flat on the top 3/4 or there was a vertical setback in the center to accommodate the width of the flathead 6.

The original frames were pretty consistent. Since the available motors were all in line straight and upright standing motors, the steering box bolted to the frame and sat about the center of the driver side of the motor. (Note most export trucks had the steering column, steering box and master cylinder all mounted on the right side of the motor as sitting in the seat, and USA trucks have the column, steering box, and master cylinder on the left side of the motor.) There were brackets that extended from each side of the frame rails towards the bellhousing and the bell was bolted to those brackets. The master cylinder mounted to driver side bell bracket.

Installing a modern motor & trans present problems in 3 basic areas.
1) Modern motors & trans are mounted with 2 front mounts and one mount at the rear of the trans. The original frame only has a center front mount. Mounts need to be made so the modern motor can be mounted on the frame rails on each side. The center mount needs to be removed, but leave the crossmember under intact.
2) The modern trans bell has no provisions for the original bell mounts. Consequently, the original brackets may be in the way of mounting the modern trans. Removing the original brackets may require a different method to mount the master cylinder. Also, the modern trans mounts with a single mount at the rear of the trans, there is no crossmember at that point to use for the rear trans mount. A crossmember and trans mount will need to be made.
3) The original position of the steering box and its attached column interferes with mounting a modern motor centered in the car. Often the modern motor & trans can be mounted offset towards the passenger side 1 1/2" to 2" to clear the steering box and the steering column. On some V8s special driver side exhaust manifolds are required to clear the steering column at the rear cylinder. There is enough movement in the u-joints to allow a 2" drive train offset.

Mixing of old technology and modern technology is always a challenging undertaking. Adopters are probably available to go either way, but once the cost is added up, the cost of keeping the old technology may not be cost, or benefit, effective.

Items required to add a modern motor & trans to an old car or truck:
1) Motor & trans.
2) Front motor mounts. Years ago, there were custom made brackets available to put just about any motor anywhere, these days, the availability of those mounts may be questionable. With some fabrication skills, the brackets are not that complex, the requirement of a "kit" is questionable.
3) A transmission crossmember and transmission mount. Again, not a difficult thing to make, or adapt from a more modern truck.
4) A rad to match you motor requirements and will fit in the grill shell. There are lots of options available.
5) Wiring required to operate your motor & trans. How complex that is, is up to you.
6) Trans yolk to fit your trans and the drive shaft u-joint.
7) Exhaust, how complex that is, is up to you.
8) Throttle & shift linkage. Adapt what you have, or build something new.
9) Fuel tank, fuel pump, fuel lines?
10) I'm sure I missed something. So this space is for that.

I also feel obligated to suggest that if your adding a modern drive train, you also upgrade your brakes. The old Mopar brakes were good for their day and the amount of power the drive trains delivered, Even the weakest modern motors exceed what was originally there. A front disc brake conversion is probably cheaper then rebuilding your original brakes, change to the dual pot master then as well. Disc brake conversion option: www.rustyhope.com If his web site doesn't list your ride, call or email him.

I'd update the rear axle too. Removing the old rear brake drums can be a major PITA, then the rear brakes are still old technology, and the gears are probably in the 391 or lower range and too light duty for much abuse. Changing the entire rear axle fixes all that at the same time. Gene