Unknown engine-need help

I bought a 69 F-100 and I am sure it has a big block in it. I have been told it looks like a 350 modified but I didn't think Ford even make one. I need to know what I can do to locate the type of engine that came with this truck when I bought it, with the engine still being in the truck...If possible.... The cab and the truck bed is a 69 make and it sits on a newer frame. My next question would be how and what is the best way to locate the type of frame it sits on. This truck is a 4x4 with many mods... I need help...

One more question...
I have a 460 out of a 1985 ecoline van....anyone know how hard it would be to swap the engine/trans? Mods etc?

Let's start from there...thanks in advance..

Pictures man we need pictures...

if it is a 69 frame----get mounts for 460 from 1974 truck---mounts and frame adapters,shim up rear trans crossmember 1/4 inch( if 2 wheel drive)

run dual exhaust down right hand side

Who ever told you 350 modified probably meant a 351M. It's a common Ford engine used in mid/full sized passenger cars and light trucks from the mid 70's up through the early 80's if I remember correctly, sharing the basic characteristics of the 400M that Ford started using in 1972. I haven't done much with any 351M/400's but I believe they have the same bellhousing bolt pattern as the 429/460.

As far as what the truck would have came with originally back in 1969, the three common V-8's would have been a 302 windsor small block, a 360 FE big block, or a 390 FE big block.