I do not know which model of Diesel is in your plans. If you are thinking of using a CUMMINS b series, I have both the 4bta and the isb 5.9 inline 6 in the shop. If you need any diminutions and pictures I can get these onto the forum for you.
My son and I have an L110 build in the works that includes a CUMMINS 4bta with a TH475, transfer case and DANA axles. The 4bta is nearly a clear fit and the only clearance issue is about .375" for the starter, on the right side. Lots of pictures can be viewed in the L,R,S build off section of this forum.
I have a local acquaintance who likes the "L" series Binders. He likes the isb, 5.9 Cummins inline 6, commonly found in the RAM trucks. I know the CUMMINS 5.9 six will just fit with a little clearance left over, in an "L". This means nothing to your situation.
In 1990, I installed an 6.9 DIESEL V8 into an R120. There was a serious clearance situation that required the firewall to be set back to accommodate the 37.5 inch wide V8 engine. The set-back was just to the original factory set-back for the SD220. I had to open and step out a section of the left inner fender apron for side clearance, so I installed a shielded power steering oil cooer in the opening. The front to back measurement of the 6,9 idi was the same as the original 220SD and then everything else changed. The set back modification was just hours of my time and not all that expensive. In the end, the accelerator pedal was in the original location.
A D30 looks like a substantial truck and may have a stronger frame. The frames on the later binders did not have any real frame strength in the R120 and lighter duty models. This is contrary to the BS that is being promoted. The added costs of getting this great lump V8 Diesel in and keeping it there was frame and suspension modifications. If I had to do it all over again, I would level the "R" frame on adjustable trestles (aka steel saw-horses) and use the IH frame as a build template for a custom frame. Here is something to think about. Our CUMMINS 4bta has more performance than the 6.9idi V8 and weighs almost 500 pounds less.
After having considered all that I have become familiar with, I have to say that an inline engine is a better and easier fit in an OLHIHC. The 6.9idi and later 7.3 idi engines are not very powerful are very heavy and tend to leak oil. The 7.3 Powerstroke needs more clearance and fitting is much more difficult. If you do know that a 7.3 Powerstroke has had a really good service history, you may be buying a seriously expensive needs-all-engine that could easily have a repair bill of between $7,000 to $17,000.
I like your project and the style of the "D" series.
There are a few interesting Japanese turbo Diesel engines in PUD trucks running the streets in this area. They all have automatics and the performance seems to be really good. I am mildly curious but not enough to go for a look. In this area, we have 99 Truck Parts and their selection of engines and transmissions is fantastic.
http://youtu.be/piPq01w1YHo