The 05-42MS05 mower deck is a 1986-88 42" side discharge. The "42" and "S" tell us that. The 74883 is the serial number and suspect it was made in 1987 as there does not appear to be any made in 1988 but they sold them in 1988 so likely overstock. You can download the operator manual fromhttps://lookup3.toro.com/request/request.cfm
Enter the 05-42MS05 model number and type the word none in the serial # box. For parts lists the best are herehttp://www.partstree.com/parts/
Type Toro in the 2nd box and the 05-42MS05 in the 3rd box. The parts that are still available from Toro are priced.
The mower drive belt is 102742 replaced by Toro 94-2501. [6378 Gates 1/2"x103-1/4" 40 degrees] [6378 Napa 1/2"x103-1/4" 40 degrees]
The spindle belt is Toro 6738 [6886 Gates 1/2"x86"] [4L860W Napa 1/2"x86"]
My first choice would be original Toro belts but the others will work. They are a special design to withstand the reverse bending over the flat idler pulleys.
The 416-8 was made 1985-1997. Post all the numbers you can find on the engine data decal/plate and will try to find you manuals for it. They used a 16hp Kohler single-cylinder engine and a twin-cylinder Onan in the later years. Can't go by the mower deck because they changed very little and fit 1970's through the last GT's in 2007.