Tough call, Mike.
Too bad they don't show photos of the decals on their web site.
The RI maroon & shield scheme could be found anytime between 1960 to even as late as 1980 (some locos just never went to a newer scheme!).
The RI GP35 era (34 of them!) was from 1965 (#305 bought with the first bunch in April of that year) and all except #306 & #310 stayed in service until shutdown in 1980. The first 22 were all bought in the M & S scheme.
RI305 was later repainted in the maroon & yellow scheme, according to Jeff's site. I don't know the date, but the M&Y dates from 1966. #305 stayed in M&Y until sold in 1980.
So, we are looking at a mid-to-late-60s loco, here.
Alas, all this data is not much help with the decals, as both cover that period. MS#87-361 is for maroon, but for CAB units. The GP35 is a hood unit.
I would be tempted to go with #87-18 (dsl, 63-75).
I would be even more tempted to call or e-mail Microscale and ask them what the decals are like and if they have a recommendation! Of course, the main thing you need is the red shields.
Or, you could just buy them both and save the leftovers for a future model! Or send any you don't want to me ....
Sorry for a lot of words, but little help!
BTW, are you using an Athearn GP35 or what ???
- George